minor cleanup on docked_magnifier_controller declaration

TBR=sky@chromium.org, afakhry@chromium.org

Bug: 958197
Test: build
Change-Id: Ie871518604b9ed74f7cb8a3f1a6bd34d2b7aefc3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1615597Reviewed-by: Jun Mukai <mukai@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660635}

ash/public/cpp/docked_magnifier_controller.h

@@ -6,7 +6,10 @@
 #define ASH_PUBLIC_CPP_DOCKED_MAGNIFIER_CONTROLLER_H_
 
 #include "ash/ash_export.h"
-#include "ui/gfx/geometry/point.h"
+
+namespace gfx {
+class Point;
+}
 
 namespace ash {