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

third_party/WebKit/Source/modules/compositorworker/CompositorWorker.cpp

Index: third_party/WebKit/Source/modules/compositorworker/CompositorWorker.cpp
diff --git a/third_party/WebKit/Source/modules/compositorworker/CompositorWorker.cpp b/third_party/WebKit/Source/modules/compositorworker/CompositorWorker.cpp
index 3eefc6ac2809d657c9ed86b7a22aaaa1ff712249..73b3fa2f5d441616eb00aa0d60b8e1c5b0806fbc 100644
--- a/third_party/WebKit/Source/modules/compositorworker/CompositorWorker.cpp
+++ b/third_party/WebKit/Source/modules/compositorworker/CompositorWorker.cpp
@@ -10,6 +10,7 @@
 #include "core/workers/WorkerClients.h"
 #include "modules/EventTargetModules.h"
 #include "modules/compositorworker/CompositorWorkerMessagingProxy.h"
+#include "modules/compositorworker/CompositorWorkerThread.h"
 
 namespace blink {
 
@@ -20,6 +21,9 @@ inline CompositorWorker::CompositorWorker(ExecutionContext* context)
 
 CompositorWorker* CompositorWorker::create(ExecutionContext* context, const String& url, ExceptionState& exceptionState)
 {
+    // Ensure the compositor worker backing thread is ready.
+    CompositorWorkerThread::ensureSharedBackingThread();

[haraken, 2016/05/09 02:00:53]

Move this just before worker->initialize? At least, it doesn't need to be
initialized until we call worker->initialize.

[flackr, 2016/05/10 18:07:39]

Unless I'm mistaken, we have to ensure the shared backing thread is created and
an initialize task posted before calling worker->initialize because when we
create the worker client we need to create CompositorMutatorImpl (GC'd class) on
the compositor thread. This all happens before the worker is created so the
thread needs to have been attached before we can do the worker initialization.

You can see this at https://codereview.chromium.org/1895873006 in
createCompositorProxyClient (called during worker initialization).

I've added to the comment to explain this.

+
     ASSERT(isMainThread());
     Document* document = toDocument(context);
     if (!document->page()) {