Add comment to PPB_MouseInputEvent version 1.1 that GetMovement hasn't been supported.

ppapi/cpp/input_event.h

Index: ppapi/cpp/input_event.h
diff --git a/ppapi/cpp/input_event.h b/ppapi/cpp/input_event.h
index d64e826882839dc606307d5cb571642a42931a56..da92742c3b026bcfe934a2fcdb572520c40c536a 100644
--- a/ppapi/cpp/input_event.h
+++ b/ppapi/cpp/input_event.h
@@ -159,6 +159,10 @@ class MouseInputEvent : public InputEvent {
   ///
   /// @return The change in position of the mouse, relative to the previous
   /// position.
+  ///
+  /// TODO(yzshen): This feature hasn't been supported yet. The returned value
+  /// is always (0, 0) for system-generated mouse events (which are passed
+  /// through the <code>HandleInputEvent</code>).
   Point GetMovement() const;
 };