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

Source/platform/WebThreadSupportingGC.h

Index: Source/platform/WebThreadSupportingGC.h
diff --git a/Source/platform/WebThreadSupportingGC.h b/Source/platform/WebThreadSupportingGC.h
index 25e831e18d467f83e20c38afc7da98330f1790ab..53c74d3a039c057abe5f01d661e75358b4df430c 100644
--- a/Source/platform/WebThreadSupportingGC.h
+++ b/Source/platform/WebThreadSupportingGC.h
@@ -15,6 +15,17 @@
 
 namespace blink {
 
+class PLATFORM_EXPORT GCSupportForWebThread final {
+public:
+    static PassOwnPtr<GCSupportForWebThread> create(WebThread&);
+    ~GCSupportForWebThread();
+
+private:
+    explicit GCSupportForWebThread(WebThread&);
+    WebThread& m_thread;
+    OwnPtr<PendingGCRunner> m_pendingGCRunner;
+};
+
 // WebThreadSupportingGC wraps a WebThread and adds support for attaching
 // to and detaching from the Blink GC infrastructure. The initialize method
 // must be called during initialization on the WebThread and before the
@@ -64,7 +75,7 @@ public:
 private:
     explicit WebThreadSupportingGC(const char*);
 
-    OwnPtr<PendingGCRunner> m_pendingGCRunner;
+    OwnPtr<GCSupportForWebThread> m_gcSupport;
 
     // FIXME: This has to be last because of crbug.com/401397.
     // A WorkerThread might get deleted before it had a chance to properly