Oilpan: Fix threading races on ThreadState::s_mainThreadStackStart

Source/platform/heap/ThreadState.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 349 matching lines @@
 
 ThreadState::~ThreadState()
 {
     checkThread();
     delete m_weakCallbackStack;
     m_weakCallbackStack = 0;
     for (int i = 0; i < NumberOfHeaps; i++)
         delete m_heaps[i];
     deleteAllValues(m_interruptors);
     **s_threadSpecific = 0;
-    s_mainThreadStackStart = 0;
-    s_mainThreadUnderestimatedStackSize = 0;
+    if (isMainThread()) {
+        s_mainThreadStackStart = 0;
+        s_mainThreadUnderestimatedStackSize = 0;
+    }
 }
 
 void ThreadState::init()
 {
     s_threadSpecific = new WTF::ThreadSpecific<ThreadState*>();
     s_safePointBarrier = new SafePointBarrier;
 }
 
 void ThreadState::shutdown()
 {
@@ skipping 891 matching lines @@
             return gcInfo;
         }
     }
     if (needLockForIteration)
         threadAttachMutex().unlock();
     return 0;
 }
 #endif
 
 }