compositor-worker: Get the thread to run compositor-workers from the Platform.

Source/modules/compositorworker/CompositorWorkerManager.h

Index: Source/modules/compositorworker/CompositorWorkerManager.h
diff --git a/Source/modules/compositorworker/CompositorWorkerManager.h b/Source/modules/compositorworker/CompositorWorkerManager.h
index aeafa08fc8825a8a89ae432c79945c6da99c40bf..f5a3eae1fb29d92cb2dbaf987b1db03c24e3afb9 100644
--- a/Source/modules/compositorworker/CompositorWorkerManager.h
+++ b/Source/modules/compositorworker/CompositorWorkerManager.h
@@ -13,8 +13,9 @@
 
 namespace blink {
 
+class GCSupportForWebThread;
 class V8IsolateInterruptor;
-class WebThreadSupportingGC;
+class WebThread;
 
 class MODULES_EXPORT CompositorWorkerManager final {
 public:
@@ -25,7 +26,7 @@ public:
 
     // Returns the thread used for compositor workers. This creates a new thread if a
     // thread doesn't already exist.
-    WebThreadSupportingGC& compositorWorkerThread();
+    WebThread& compositorWorkerThread();
 
     // Attempts to initialize/shutdown a thread if necessary. Does nothing if the thread
     // is already initialized, or if the thread has more than one active workers at the
@@ -45,7 +46,8 @@ private:
     ~CompositorWorkerManager();
 
     Mutex m_mutex;
-    OwnPtr<WebThreadSupportingGC> m_thread;
+    OwnPtr<WebThread> m_thread;
+    OwnPtr<GCSupportForWebThread> m_gcSupport;
     int m_workerCount = 0;
     v8::Isolate* m_isolate = nullptr;
 };