Add missing base/memory/ptr_util.h inclusion to palette_tray_unittest.cc

This file indirectly depends on ptr_util.h inclusion in
cc/resources/single_release_callback.h, which is being removed.

Tbr: sadrul@chromium.org
Change-Id: I3fd1b38c4b8808bdb67001756b2d60416e6bb756
Reviewed-on: https://chromium-review.googlesource.com/579228Reviewed-by: Taiju Tsuiki <tzik@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488225}

ash/system/palette/palette_tray_unittest.cc

@@ -11,6 +11,7 @@
 #include "ash/system/status_area_widget_test_helper.h"
 #include "ash/test/ash_test_base.h"
 #include "base/command_line.h"
+#include "base/memory/ptr_util.h"
 #include "ui/events/event.h"
 
 namespace ash {