exo: Use DESCENDANTS_ONLY event targeting policy for mus/mash

TARGET_AND_DESCENDANTS event targeting policy does not work properly
under mus/mash. As a result, mouse events are not delivered to
the windows under ExoShellSurface in SystemModalContainer even though
ExoShellSurface says it does not handle the events.

Use DESCENDANTS_ONLY for mus/mash until event dispatching code is
fixed to handle such case.

Bug: 839521
Change-Id: I8e949164d2cd9666468a8995d0bdb8e29f6b9301
Reviewed-on: https://chromium-review.googlesource.com/1044542
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: David Reveman <reveman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556176}
1 file changed