Add start/stop listening Platform methods for Screen Orientation.

public/platform/Platform.h

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 622 matching lines @@
 
     // WebDatabase --------------------------------------------------------
 
     virtual WebDatabaseObserver* databaseObserver() { return 0; }
 
 
     // Web Notifications --------------------------------------------------
 
     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() { }
+

[abarth-chromium, 2014/07/16 17:12:17]

The way we've done this for other APIs is to set a listener pointer to a null or
non-null value.

It's a bit strange for these calls to be routed through the platform but for the
notification to come in from the web layer.  Can we signal this information via
the web frame client?  In principle, Platform and WebFrameClient are completely
separate systems.

 
 protected:
     virtual ~Platform() { }
 };
 
 } // namespace blink
 
 #endif