Change WokerThread to use a blink::WebThread.

public/platform/WebThread.h

Index: public/platform/WebThread.h
diff --git a/public/platform/WebThread.h b/public/platform/WebThread.h
index 6bee10fbfebd929305b7c94031e4bf87e545f79b..f1a47465730fe24cae3ffcdeb55a8a2151a09577 100644
--- a/public/platform/WebThread.h
+++ b/public/platform/WebThread.h
@@ -68,6 +68,12 @@ public:
     // Must be called when the WebThread is running.
     virtual void exitRunLoop() = 0;
 
+    // Delayed work is driven by a shared timer.
+    typedef void (*SharedTimerFunction)();
+    virtual void setSharedTimerFiredFunction(SharedTimerFunction) = 0;
+    virtual void setSharedTimerFireInterval(double) = 0;
+    virtual void stopSharedTimer() = 0;
+
     virtual ~WebThread() { }
 };