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() { }
+

[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() { }