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

Source/core/workers/WorkerThreadTest.cpp

Index: Source/core/workers/WorkerThreadTest.cpp
diff --git a/Source/core/workers/WorkerThreadTest.cpp b/Source/core/workers/WorkerThreadTest.cpp
index f0e7725a9f4825755b092979da7275632736e04c..f33e4f0f999237cd17ce4da1b7e8b33f8667d794 100644
--- a/Source/core/workers/WorkerThreadTest.cpp
+++ b/Source/core/workers/WorkerThreadTest.cpp
@@ -97,6 +97,15 @@ public:
         return *m_thread;
     }
 
+    OwnPtr<WorkerV8Isolate>& workerIsolate() override
+    {
+        if (!m_isolate) {
+            ASSERT(isCurrentThread());
+            m_isolate = WorkerV8Isolate::createDefault();
+        }
+        return m_isolate;
+    }
+
     MOCK_METHOD1(doIdleGc, bool(double deadlineSeconds));
 
     PassRefPtrWillBeRawPtr<WorkerGlobalScope> createWorkerGlobalScope(PassOwnPtr<WorkerThreadStartupData> startupData) override
@@ -106,6 +115,7 @@ public:
 
 private:
     OwnPtr<WebThreadSupportingGC> m_thread;
+    OwnPtr<WorkerV8Isolate> m_isolate;
 };
 
 class WakeupTask : public WebThread::Task {