compositor-worker: Share a thread and an isolate for compositor workers.

Source/core/workers/WorkerThread.h

Index: Source/core/workers/WorkerThread.h
diff --git a/Source/core/workers/WorkerThread.h b/Source/core/workers/WorkerThread.h
index 0753bc3b766c0ec862f00feaafe5d1c87020077d..c336ac9fe138f5fac3e7fe9b4a4284d9b9467dc7 100644
--- a/Source/core/workers/WorkerThread.h
+++ b/Source/core/workers/WorkerThread.h
@@ -32,6 +32,7 @@
 #include "core/frame/csp/ContentSecurityPolicy.h"
 #include "core/workers/WorkerGlobalScope.h"
 #include "core/workers/WorkerLoaderProxy.h"
+#include "core/workers/WorkerV8Isolate.h"
 #include "platform/WebThreadSupportingGC.h"
 #include "platform/weborigin/SecurityOrigin.h"
 #include "wtf/Forward.h"
@@ -71,8 +72,10 @@ public:
 
     virtual void didStartRunLoop();
     virtual void didStopRunLoop();
+    virtual void initializeBackingThread();
+    virtual void shutdownBackingThread();
 
-    v8::Isolate* isolate() const { return m_isolate; }
+    v8::Isolate* isolate() const { return m_isolate ? m_isolate->isolate() : nullptr; }
 
     // Can be used to wait for this worker thread to shut down.
     // (This is signalled on the main thread, so it's assumed to be waited on the worker context thread)
@@ -121,7 +124,7 @@ protected:
 
     virtual void postInitialize() { }
 
-    virtual v8::Isolate* initializeIsolate();
+    virtual PassOwnPtr<WorkerV8Isolate> initializeIsolate();
     virtual void willDestroyIsolate();
     virtual void destroyIsolate();
     virtual void terminateV8Execution();
@@ -153,13 +156,12 @@ private:
     RefPtrWillBePersistent<WorkerInspectorController> m_workerInspectorController;
     Mutex m_workerInspectorControllerMutex;
 
-    // This lock protects |m_workerGlobalScope|, |m_terminated|, |m_isolate| and |m_microtaskRunner|.
+    // This lock protects |m_workerGlobalScope|, |m_terminated|, and |m_isolate|.
     Mutex m_threadStateMutex;
 
     RefPtrWillBePersistent<WorkerGlobalScope> m_workerGlobalScope;
 
-    v8::Isolate* m_isolate;
-    OwnPtr<V8IsolateInterruptor> m_interruptor;
+    OwnPtr<WorkerV8Isolate> m_isolate;
 
     // Used to signal thread shutdown.
     OwnPtr<WebWaitableEvent> m_shutdownEvent;