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

ppapi/c/ppb_input_event.h

Index: ppapi/c/ppb_input_event.h
diff --git a/ppapi/c/ppb_input_event.h b/ppapi/c/ppb_input_event.h
index 957694cea521aebb5c1d51c3fa7bb9caff312f89..8a73279e4ebc51591764733afdd94b3da838d0f6 100644
--- a/ppapi/c/ppb_input_event.h
+++ b/ppapi/c/ppb_input_event.h
@@ -475,6 +475,10 @@ struct PPB_MouseInputEvent {
    *
    * @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>).
    */
   struct PP_Point (*GetMovement)(PP_Resource mouse_event);
 };