The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a98c735e607bb98df82999619a0fdb392af048b3

commit a98c735e607bb98df82999619a0fdb392af048b3
Author: Scott Violet <sky@chromium.org>
Date: Tue Jul 31 19:38:56 2018

aura: cleanup mus related test helper

AuraTestHelper now always create a WindowTreeHost. This resulted in being
able to remove some test code around configuring when to create the
WindowTreeHost. The only ick that was needed to support this is in AxRemoteHost.
AxRemoteHost assumed there was only one root, which isn't the case in test
code anymore. I'm hoping that once we make AxRemoteHost support more than one
root this code will go away.

There was also some mild trickery around window ids. In particular
WindowTreeClientPrivate has some functions that pass ids to WindowTreeClient.
These ids are used in creating windows, and must be unique. Because
AuraTestHelper now always triggers creating a WindowTreeHost, the id must be
increased. I've hopefully done things so tests shouldn't have to worry about
this.

BUG= 866634 
TEST=covered by tests

Change-Id: Ia3d2cd757555478f3cce8929574bc8df8acb3af2
Reviewed-on: https://chromium-review.googlesource.com/1153773
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579531}
[modify] https://crrev.com/a98c735e607bb98df82999619a0fdb392af048b3/ui/aura/mus/drag_drop_controller_mus_unittest.cc
[modify] https://crrev.com/a98c735e607bb98df82999619a0fdb392af048b3/ui/aura/mus/window_tree_client_unittest.cc
[modify] https://crrev.com/a98c735e607bb98df82999619a0fdb392af048b3/ui/aura/test/aura_mus_test_base.cc
[modify] https://crrev.com/a98c735e607bb98df82999619a0fdb392af048b3/ui/aura/test/aura_test_base.cc
[modify] https://crrev.com/a98c735e607bb98df82999619a0fdb392af048b3/ui/aura/test/aura_test_base.h
[modify] https://crrev.com/a98c735e607bb98df82999619a0fdb392af048b3/ui/aura/test/aura_test_helper.cc
[modify] https://crrev.com/a98c735e607bb98df82999619a0fdb392af048b3/ui/aura/test/aura_test_helper.h
[modify] https://crrev.com/a98c735e607bb98df82999619a0fdb392af048b3/ui/aura/test/mus/window_tree_client_private.cc
[modify] https://crrev.com/a98c735e607bb98df82999619a0fdb392af048b3/ui/aura/test/mus/window_tree_client_private.h
[modify] https://crrev.com/a98c735e607bb98df82999619a0fdb392af048b3/ui/views/mus/ax_remote_host.cc