Allow creating non-thread-local Persistents that can be passed between threads.

Source/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 20 matching lines @@
 #ifndef ThreadState_h
 #define ThreadState_h
 
 #include "heap/AddressSanitizer.h"
 #include "heap/HeapExport.h"
 #include "wtf/HashSet.h"
 #include "wtf/OwnPtr.h"
 #include "wtf/PassOwnPtr.h"
 #include "wtf/ThreadSpecific.h"
 #include "wtf/Threading.h"
+#include "wtf/ThreadingPrimitives.h"
 #include "wtf/Vector.h"
 
 namespace WebCore {
 
 class BaseHeap;
 class BaseHeapPage;
 class FinalizedHeapObjectHeader;
 class HeapContainsCache;
 class HeapObjectHeader;
 class PersistentNode;
@@ skipping 395 matching lines @@
     bool contains(const void* pointer) { return contains(const_cast<void*>(pointer)); }
 
     // Finds the Blink HeapPage in this thread-specific heap
     // corresponding to a given address. Return 0 if the address is
     // not contained in any of the pages.
     BaseHeapPage* heapPageFromAddress(Address);
 
     // List of persistent roots allocated on the given thread.
     PersistentNode* roots() const { return m_persistents; }
 
+    // List of global persistent roots not owned by any particular thread.
+    // globalRootsMutex must be acquired before any modifications.
+    static PersistentNode* globalRoots();
+    static Mutex& globalRootsMutex();
+
     // Visit local thread stack and trace all pointers conservatively.
     void visitStack(Visitor*);
 
     // Visit all persistents allocated on this thread.
     void visitPersistents(Visitor*);
 
     // Checks a given address and if a pointer into the oilpan heap marks
     // the object to which it points.
     bool checkAndMarkPointer(Visitor*, Address);
 
@@ skipping 85 matching lines @@
 USED_FROM_MULTIPLE_THREADS(SubtleCrypto);
 USED_FROM_MULTIPLE_THREADS(TextDecoder);
 USED_FROM_MULTIPLE_THREADS(TextEncoder);
 USED_FROM_MULTIPLE_THREADS(WebKitNotification);
 USED_FROM_MULTIPLE_THREADS(WorkerCrypto);
 USED_FROM_MULTIPLE_THREADS(WorkerPerformance);
 
 }
 
 #endif // ThreadState_h