[isolate] Remove unused save_context field

R=mstarzinger@chromium.org

Bug: v8:8562
Change-Id: I05abb6d9e594c797cad558befac4376b376bc514
Reviewed-on: https://chromium-review.googlesource.com/c/1448274Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59267}

src/isolate.cc

@@ -4532,19 +4532,16 @@ bool StackLimitCheck::JsHasOverflowed(uintptr_t gap) const {
   return GetCurrentStackPosition() - gap < stack_guard->real_climit();
 }
 
-SaveContext::SaveContext(Isolate* isolate)
-    : isolate_(isolate), prev_(isolate->save_context()) {
+SaveContext::SaveContext(Isolate* isolate) : isolate_(isolate) {
   if (!isolate->context().is_null()) {
     context_ = Handle<Context>(isolate->context(), isolate);
   }
-  isolate->set_save_context(this);
 
   c_entry_fp_ = isolate->c_entry_fp(isolate->thread_local_top());
 }
 
 SaveContext::~SaveContext() {
   isolate_->set_context(context_.is_null() ? Context() : *context_);
-  isolate_->set_save_context(prev_);
 }
 
 bool SaveContext::IsBelowFrame(StandardFrame* frame) {

src/isolate.h

@@ -402,7 +402,6 @@ class ThreadLocalTop {
   // unify them later.
   Object scheduled_exception_;
   bool external_caught_exception_ = false;
-  SaveContext* save_context_ = nullptr;
 
   // Stack.
   // The frame pointer of the top c entry frame.
@@ -672,8 +671,6 @@ class Isolate final : private HiddenFactory {
   inline void set_context(Context context);
   Context* context_address() { return &thread_local_top_.context_; }
 
-  THREAD_LOCAL_TOP_ACCESSOR(SaveContext*, save_context)
-
   // Access to current thread id.
   THREAD_LOCAL_TOP_ACCESSOR(ThreadId, thread_id)
 
@@ -1947,16 +1944,12 @@ class PromiseOnStack {
 };
 
 
-// If the GCC version is 4.1.x or 4.2.x an additional field is added to the
-// class as a work around for a bug in the generated code found with these
-// versions of GCC. See V8 issue 122 for details.
 class V8_EXPORT_PRIVATE SaveContext {
  public:
   explicit SaveContext(Isolate* isolate);
   ~SaveContext();
 
   Handle<Context> context() { return context_; }
-  SaveContext* prev() { return prev_; }
 
   // Returns true if this save context is below a given JavaScript frame.
   bool IsBelowFrame(StandardFrame* frame);
@@ -1964,7 +1957,6 @@ class V8_EXPORT_PRIVATE SaveContext {
  private:
   Isolate* const isolate_;
   Handle<Context> context_;
-  SaveContext* const prev_;
   Address c_entry_fp_;
 };