Sample: https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8933231905190382448/+/steps/single_process_mash_interactive_ui_tests/0/logs/TabDragging__x2f_DetachToBrowserTabDragControllerTest.DragToSeparateWindow__x2f_0/0

Seems there's some bug in TopmostWindowObserver. So it would be my bug...

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b417202473ac5b5cd16d41cbe4e71152747855bf

commit b417202473ac5b5cd16d41cbe4e71152747855bf
Author: Jun Mukai <mukai@chromium.org>
Date: Wed Oct 10 21:45:20 2018

Reform TopmostWindowObserver's Add/RemoveObserver

It seems current AddObserver / RemoveObserver logic is wrong and
that seems causing test flakiness of  crbug.com/893187 .

This CL fixes the logic a bit to be more redundant. Added a new
test file to cover those patterns.

BUG= 893187 
TEST=services_unittests covers

Change-Id: I33193e174bc72fc82d9c19990face9a59ca545c8
Reviewed-on: https://chromium-review.googlesource.com/c/1269999
Commit-Queue: Jun Mukai <mukai@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598521}
[modify] https://crrev.com/b417202473ac5b5cd16d41cbe4e71152747855bf/services/ws/BUILD.gn
[modify] https://crrev.com/b417202473ac5b5cd16d41cbe4e71152747855bf/services/ws/test_window_service_delegate.cc
[modify] https://crrev.com/b417202473ac5b5cd16d41cbe4e71152747855bf/services/ws/test_window_service_delegate.h
[modify] https://crrev.com/b417202473ac5b5cd16d41cbe4e71152747855bf/services/ws/topmost_window_observer.cc
[modify] https://crrev.com/b417202473ac5b5cd16d41cbe4e71152747855bf/services/ws/topmost_window_observer.h
[add] https://crrev.com/b417202473ac5b5cd16d41cbe4e71152747855bf/services/ws/topmost_window_observer_unittest.cc