[config] Add V8_NODISCARD to Scopes in v8.h

Like  https://crrev.com/c/v8/v8/+/2555001 for v8.h. Done in a separate
CL in case it needed to be reverted.

Change-Id: I0c7a7cb24e8f0855c8b80ddeeaab979f10011c4a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2562252Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71455}

include/v8.h

@@ -1197,7 +1197,7 @@ class TracedReference : public BasicTracedReference<T> {
  * handle and may deallocate it.  The behavior of accessing a handle
  * for which the handle scope has been deleted is undefined.
  */
-class V8_EXPORT HandleScope {
+class V8_EXPORT V8_NODISCARD HandleScope {
  public:
   explicit HandleScope(Isolate* isolate);
 
@@ -1244,12 +1244,11 @@ class V8_EXPORT HandleScope {
   friend class Context;
 };
 
-
 /**
  * A HandleScope which first allocates a handle in the current scope
  * which will be later filled with the escape value.
  */
-class V8_EXPORT EscapableHandleScope : public HandleScope {
+class V8_EXPORT V8_NODISCARD EscapableHandleScope : public HandleScope {
  public:
   explicit EscapableHandleScope(Isolate* isolate);
   V8_INLINE ~EscapableHandleScope() = default;
@@ -1290,7 +1289,7 @@ class V8_EXPORT EscapableHandleScope : public HandleScope {
  * are allowed. It can be useful for debugging handle leaks.
  * Handles can be allocated within inner normal HandleScopes.
  */
-class V8_EXPORT SealHandleScope {
+class V8_EXPORT V8_NODISCARD SealHandleScope {
  public:
   explicit SealHandleScope(Isolate* isolate);
   ~SealHandleScope();
@@ -1311,7 +1310,6 @@ class V8_EXPORT SealHandleScope {
   int prev_sealed_level_;
 };
 
-
 // --- Special objects ---
 
 /**
@@ -7590,7 +7588,7 @@ class V8_EXPORT MicrotaskQueue {
  * kDoNotRunMicrotasks should be used to annotate calls not intended to trigger
  * microtasks.
  */
-class V8_EXPORT MicrotasksScope {
+class V8_EXPORT V8_NODISCARD MicrotasksScope {
  public:
   enum Type { kRunMicrotasks, kDoNotRunMicrotasks };
 
@@ -7623,7 +7621,6 @@ class V8_EXPORT MicrotasksScope {
   bool run_;
 };
 
-
 // --- Failed Access Check Callback ---
 typedef void (*FailedAccessCheckCallback)(Local<Object> target,
                                           AccessType type,
@@ -8415,7 +8412,7 @@ class V8_EXPORT Isolate {
    * Stack-allocated class which sets the isolate for all operations
    * executed within a local scope.
    */
-  class V8_EXPORT Scope {
+  class V8_EXPORT V8_NODISCARD Scope {
    public:
     explicit Scope(Isolate* isolate) : isolate_(isolate) {
       isolate->Enter();
@@ -8431,11 +8428,10 @@ class V8_EXPORT Isolate {
     Isolate* const isolate_;
   };
 
-
   /**
    * Assert that no Javascript code is invoked.
    */
-  class V8_EXPORT DisallowJavascriptExecutionScope {
+  class V8_EXPORT V8_NODISCARD DisallowJavascriptExecutionScope {
    public:
     enum OnFailure { CRASH_ON_FAILURE, THROW_ON_FAILURE, DUMP_ON_FAILURE };
 
@@ -8453,11 +8449,10 @@ class V8_EXPORT Isolate {
     void* internal_;
   };
 
-
   /**
    * Introduce exception to DisallowJavascriptExecutionScope.
    */
-  class V8_EXPORT AllowJavascriptExecutionScope {
+  class V8_EXPORT V8_NODISCARD AllowJavascriptExecutionScope {
    public:
     explicit AllowJavascriptExecutionScope(Isolate* isolate);
     ~AllowJavascriptExecutionScope();
@@ -8478,7 +8473,7 @@ class V8_EXPORT Isolate {
    * Do not run microtasks while this scope is active, even if microtasks are
    * automatically executed otherwise.
    */
-  class V8_EXPORT SuppressMicrotaskExecutionScope {
+  class V8_EXPORT V8_NODISCARD SuppressMicrotaskExecutionScope {
    public:
     explicit SuppressMicrotaskExecutionScope(
         Isolate* isolate, MicrotaskQueue* microtask_queue = nullptr);
@@ -8502,7 +8497,7 @@ class V8_EXPORT Isolate {
    * This scope allows terminations inside direct V8 API calls and forbid them
    * inside any recursive API calls without explicit SafeForTerminationScope.
    */
-  class V8_EXPORT SafeForTerminationScope {
+  class V8_EXPORT V8_NODISCARD SafeForTerminationScope {
    public:
     explicit SafeForTerminationScope(v8::Isolate* isolate);
     ~SafeForTerminationScope();
@@ -10638,7 +10633,7 @@ class V8_EXPORT Context {
    * Stack-allocated class which sets the execution context for all
    * operations executed within a local scope.
    */
-  class Scope {
+  class V8_NODISCARD Scope {
    public:
     explicit V8_INLINE Scope(Local<Context> context) : context_(context) {
       context_->Enter();
@@ -10654,7 +10649,7 @@ class V8_EXPORT Context {
    * stack.
    * https://html.spec.whatwg.org/multipage/webappapis.html#backup-incumbent-settings-object-stack
    */
-  class V8_EXPORT BackupIncumbentScope final {
+  class V8_EXPORT V8_NODISCARD BackupIncumbentScope final {
    public:
     /**
      * |backup_incumbent_context| is pushed onto the backup incumbent settings