scheduler: Add new platform interface for implementing scheduling

public/platform/Platform.h

Index: public/platform/Platform.h
diff --git a/public/platform/Platform.h b/public/platform/Platform.h
index c319e30c3ab10f33258b52f0ef4015c0d13169d6..13a33491ac394f067e08928574379b752c827ed0 100644
--- a/public/platform/Platform.h
+++ b/public/platform/Platform.h
@@ -92,6 +92,7 @@ class WebPublicSuffixList;
 class WebRTCPeerConnectionHandler;
 class WebRTCPeerConnectionHandlerClient;
 class WebSandboxSupport;
+class WebScheduler;
 class WebSecurityOrigin;
 class WebScrollbarBehavior;
 class WebSocketHandle;
@@ -360,6 +361,8 @@ public:
     // Yield the current thread so another thread can be scheduled.
     virtual void yieldCurrentThread() { }
 
+    // May return null.
+    virtual WebScheduler* scheduler() { return 0; }
 
     // WaitableEvent -------------------------------------------------------