Makes WebThreadSupportingGC constructible for an existing thread

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..5a5d5436077d7ca4720b43a26908ef762d17b0e5 100644
--- a/Source/modules/compositorworker/CompositorWorkerManager.h
+++ b/Source/modules/compositorworker/CompositorWorkerManager.h
@@ -14,6 +14,7 @@
 namespace blink {
 
 class V8IsolateInterruptor;
+class WebThread;
 class WebThreadSupportingGC;
 
 class MODULES_EXPORT CompositorWorkerManager final {
@@ -46,6 +47,7 @@ private:
 
     Mutex m_mutex;
     OwnPtr<WebThreadSupportingGC> m_thread;
+    OwnPtr<WebThread> m_platformThread;
     int m_workerCount = 0;
     v8::Isolate* m_isolate = nullptr;
 };