compositor-worker: Keep worker backing thread alive for the lifetime of the compositor thread.

third_party/WebKit/Source/core/workers/WorkerBackingThread.h

Index: third_party/WebKit/Source/core/workers/WorkerBackingThread.h
diff --git a/third_party/WebKit/Source/core/workers/WorkerBackingThread.h b/third_party/WebKit/Source/core/workers/WorkerBackingThread.h
index 08b3fc25ce26f869ab93a104a7ae986c12c4a57d..79b6cac1204e9b6ae9f69b547f9b44af5047d989 100644
--- a/third_party/WebKit/Source/core/workers/WorkerBackingThread.h
+++ b/third_party/WebKit/Source/core/workers/WorkerBackingThread.h
@@ -14,6 +14,7 @@
 
 namespace blink {
 
+class WaitableEvent;
 class WebThread;
 class WebThreadSupportingGC;
 
@@ -65,7 +66,8 @@ private:
     WorkerBackingThread(const char* name, bool shouldCallGCOnShutdown);
     WorkerBackingThread(WebThread*, bool shouldCallGCOnSHutdown);
     void initialize();
-    void shutdown();
+    void shutdown(WaitableEvent* doneEvent = nullptr);
+    void signalShutdownAndWait();
 
     // Protects |m_workerScriptCount|.
     Mutex m_mutex;