Add start/stop listening Platform methods for Screen Orientation.

public/platform/Platform.h

Index: public/platform/Platform.h
diff --git a/public/platform/Platform.h b/public/platform/Platform.h
index 259143d042d95541d8b18bded3782dbfab592196..8e21b6d4cfedc1e62bb23bda388aee50a2463cff 100644
--- a/public/platform/Platform.h
+++ b/public/platform/Platform.h
@@ -640,6 +640,23 @@ public:
 
     virtual WebNotificationPresenter* notificationPresenter() { return 0; }
 
+    // Screen Orientation -------------------------------------------------
+
+    // Informs the platform that if it requires a specific observer to run in
+    // order to know the screen orientation accurately, it should be started.
+    // This call is expected to happen as soon as a WebFrame starts using the
+    // Screen Orientation API.
+    // Most implementations are expected to do nothing.
+    virtual void startScreenOrientationListening() { }
+
+    // Informs the platform that if it requires a specific observer to run in
+    // order to know the screen orientation, it can be stopped.
+    // This call is expected to happen as soon as a WebFrame using the Screen
+    // Orientation no longer uses it. It is up to the platform to make sure that
+    // no other consumers are still expecting accurate values.
+    // Most implementations are expected to do nothing.
+    virtual void stopScreenOrientationListening() { }
+
 
 protected:
     virtual ~Platform() { }