Fix nested parking of threads while setting up a garbage collection.

Source/platform/heap/Heap.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 1864 matching lines @@
     RELEASE_ASSERT(!state->isInGC());
     state->completeSweep();
     ThreadState::GCState originalGCState = state->gcState();
     state->setGCState(ThreadState::StoppingOtherThreads);
 
     GCScope gcScope(stackState);
     // Check if we successfully parked the other threads.  If not we bail out of
     // the GC.
     if (!gcScope.allThreadsParked()) {
         // Restore the original GCState.
-        state->setGCState(originalGCState);
+        if (LIKELY(state->gcState() == ThreadState::StoppingOtherThreads))
+            state->setGCState(originalGCState);
         return;
     }
 
     if (state->isMainThread())
         ScriptForbiddenScope::enter();
 
     s_lastGCWasConservative = false;
 
     TRACE_EVENT2("blink_gc", "Heap::collectGarbage",
         "lazySweeping", gcType == ThreadState::GCWithoutSweep,
@@ skipping 327 matching lines @@
 size_t Heap::s_allocatedObjectSize = 0;
 size_t Heap::s_allocatedSpace = 0;
 size_t Heap::s_markedObjectSize = 0;
 // We don't want to use 0 KB for the initial value because it may end up
 // triggering the first GC of some thread too prematurely.
 size_t Heap::s_estimatedLiveObjectSize = 512 * 1024;
 size_t Heap::s_externalObjectSizeAtLastGC = 0;
 double Heap::s_estimatedMarkingTimePerByte = 0.0;
 
 } // namespace blink