chromeos: change implementation of AshTestBase::CreateTestWindow()

It no longer simulates what happens with the window-service, rather it creates
a widget and/or window depending upon the parameters.

BUG=958062
TEST=test only changes

Change-Id: I132fd975209ad0a4dd268e934359e123cc4ab983
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1599875
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657420}
8 files changed