The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5f8a6330d36c11874bb8aaf136f3ea4e508ae8ee

commit 5f8a6330d36c11874bb8aaf136f3ea4e508ae8ee
Author: moshayedi <moshayedi@chromium.org>
Date: Fri Mar 10 14:51:03 2017

Fix event targeting for sub-windows of system modal windows in mus+ash.

If a non-client frame is set to a system modal, its embedded sub-window
should be able to receive events.

For example, creating a JS alert() creates a non-client frame and sets it
to MODAL_TYPE_SYSTEM. Previously users couldn't click on the OK inside
the client window since all events were routed to the non-client frame.

BUG= 699213 

Review-Url: https://codereview.chromium.org/2737003002
Cr-Commit-Position: refs/heads/master@{#456058}

[modify] https://crrev.com/5f8a6330d36c11874bb8aaf136f3ea4e508ae8ee/services/ui/ws/event_dispatcher_unittest.cc
[modify] https://crrev.com/5f8a6330d36c11874bb8aaf136f3ea4e508ae8ee/services/ui/ws/modal_window_controller.cc