[api] Add Check method to Maybe

This new method returns no value and just verifies that the Maybe is not
Empty. This is intended to be used for functions like Object::Set that
return a Maybe<bool> but only use "emptiness" to indicate its result and
where ToLocalChecked() appears to be discarding information.

Also comments Object::Set to indicate that Check() is the preferred way
of asserting that the Set should always succeed.

R=yangguo

Bug: v8:8562
Change-Id: Ic3b45e42fa9ba0b53f1a764660a56041e64f087b
Reviewed-on: https://chromium-review.googlesource.com/c/1414912Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58852}

include/v8.h

@@ -3217,6 +3217,10 @@ class V8_EXPORT Object : public Value {
  public:
   V8_DEPRECATE_SOON("Use maybe version",
                     bool Set(Local<Value> key, Local<Value> value));
+  /**
+   * Set only return Just(true) or Empty(), so if it should never fail, use
+   * result.Check().
+   */
   V8_WARN_UNUSED_RESULT Maybe<bool> Set(Local<Context> context,
                                         Local<Value> key, Local<Value> value);
 
@@ -8716,6 +8720,14 @@ class Maybe {
    */
   V8_INLINE T ToChecked() const { return FromJust(); }
 
+  /**
+   * Short-hand for ToChecked(), which doesn't return a value. To be used, where
+   * the actual value of the Maybe is not needed like Object::Set.
+   */
+  V8_INLINE void Check() const {
+    if (V8_UNLIKELY(!IsJust())) V8::FromJustIsNothing();
+  }
+
   /**
    * Converts this Maybe<> to a value of type T. If this Maybe<> is
    * nothing (empty), |false| is returned and |out| is left untouched.