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This makes AshTestBase::CreateTestWindow() get the logic (when in
mash) that is in
WmTestHelper::CreateTestWindow(). I added
AshTestBase::CreateTestWindow() for migration from AshTest/WmTestBase
to AshTestBase.

BUG=707057
TEST=test only changes
R=jamescook@chromium.org

Review-Url: https://codereview.chromium.org/2840393002
Cr-Commit-Position: refs/heads/master@{#467566}