Cheaper thread-safe atomic initialization of static references.

Source/platform/heap/ThreadState.h

 /*
  * 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 455 matching lines @@
     // heap the containing heap page is returned.
     BaseHeapPage* findPageFromAddress(Address);
     BaseHeapPage* findPageFromAddress(void* pointer) { return findPageFromAddress(reinterpret_cast<Address>(pointer)); }
 #endif
 
     // List of persistent roots allocated on the given thread.
     PersistentNode* roots() const { return m_persistents.get(); }
 
     // List of global persistent roots not owned by any particular thread.
     // globalRootsMutex must be acquired before any modifications.
-    static PersistentNode* globalRoots();
+    static PersistentNode& globalRoots();
     static Mutex& globalRootsMutex();
 
     // Visit local thread stack and trace all pointers conservatively.
     void visitStack(Visitor*);
 
     // Visit the asan fake stack frame corresponding to a slot on the
     // real machine stack if there is one.
     void visitAsanFakeStackForPointer(Visitor*, Address);
 
     // Visit all persistents allocated on this thread.
@@ skipping 232 matching lines @@
         m_locked = false;
     }
 
     MutexBase& m_mutex;
     bool m_locked;
 };
 
 } // namespace blink
 
 #endif // ThreadState_h