Remove did(Start|Stop)WorkerThread as it is not used.

public/platform/Platform.h

Index: public/platform/Platform.h
diff --git a/public/platform/Platform.h b/public/platform/Platform.h
index a435b941113414d1affd31281b190af08ed7caab..7a2841dff4716260a81d12150a653517dbf64597 100644
--- a/public/platform/Platform.h
+++ b/public/platform/Platform.h
@@ -597,9 +597,6 @@ public:
     virtual void didStartWorkerRunLoop(const WebWorkerRunLoop&) { }
     virtual void didStopWorkerRunLoop(const WebWorkerRunLoop&) { }
 
-    virtual void didStartWorkerThread(WebThread*) { }
-    virtual void didStopWorkerThread(WebThread*) { }
-
     // WebCrypto ----------------------------------------------------------
 
     virtual WebCrypto* crypto() { return 0; }