Do not handle ui::ET_GESTURE_END events in CustomButton

Do not handle ui::ET_GESTURE_END events in CustomButton

We plan on no longer dispatching GESTURE_END events
to views (crbug.com/397564), so do not handle this
event type in CustomButton::OnGestureEvent().
This event is currently used to start a fade-out
animation on a TAP gesture, but we can accomplish
the same goal by starting this animation when
receiving the TAP gesture itself.

BUG=397579
TEST=MenuButtonTest.ActivateNonDropDownOnGestureTap

[tdanderson, 2014-07-25 18:50:54]

Sadrul, can you please take a look? This preserves the existing visual
behaviour.

[sadrul, 2014-07-25 20:39:41]

lgtm

[tdanderson, 2014-07-25 20:43:24]

The CQ bit was checked by tdanderson@chromium.org

[commit-bot: I haz the power, 2014-07-25 20:45:17]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/tdanderson@chromium.org/416343003/1

[commit-bot: I haz the power, 2014-07-25 21:49:14]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  win_chromium_compile_dbg on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_compile_...)
  win_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_rel/buil...)

[tdanderson, 2014-07-25 22:16:52]

The CQ bit was unchecked by tdanderson@chromium.org