GitLab

Prior to this patch AuraTestHelper configured TestScreen in such a way
that it would create a WindowTreeHost to represent a screen. The
WindowTreeHost was created was not a WindowTreeHostMus, rather a
WindowTreeHostPlatform. This doesn't make sense for mus.

As part of this I'm also moving how ash configures Env to
AuraTestBase. This way other code using AuraTestBase can work better
with mus.

Lastly I needed to make sure sure AuraTestHelper resets some state
that was persisting between runs. This is necessitated by Env being
created at the TestSuite level, not per test.

BUG=776514
TEST=covered by tests

Change-Id: Icb1b5342f962f44c03185a3e40097491532c4efe
Reviewed-on: https://chromium-review.googlesource.com/791821Reviewed-by: Elliot Glaysher <erg@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519758}