Fixed the ScreenOrientationController so that it doesn't crash by animating

ash/rotator/screen_rotation_animator.h

Index: ash/rotator/screen_rotation_animator.h
diff --git a/ash/rotator/screen_rotation_animator.h b/ash/rotator/screen_rotation_animator.h
index 055e755906dc483d95f4da29cc666749d893eb66..5470d0ae235986509b24d1564c027d0c6526a242 100644
--- a/ash/rotator/screen_rotation_animator.h
+++ b/ash/rotator/screen_rotation_animator.h
@@ -17,7 +17,15 @@ class ASH_EXPORT ScreenRotationAnimator {
   explicit ScreenRotationAnimator(int64 display_id);
   ~ScreenRotationAnimator();
 
-  // Rotates |display_| to the |new_rotation| orientation.
+  // Returns true if the screen rotation animation can be completed
+  // successfully. For example an animation is not possible if |display_id_|
+  // specifies a gfx::Display that is not currently active. See
+  // www.crbug.com/479503.
+  bool CanAnimate() const;
+
+  // Rotates the gfx::Display specified by |display_id_| to the |new_rotation|
+  // orientation. Clients should only call |Rotate(gfx::Display::Rotation)| if
+  // |CanAnimate()| returns true.
   void Rotate(gfx::Display::Rotation new_rotation);
 
  private: