Run mash_content_unittests on bots

- Add mash_content_unittests that runs content_unittests under mash;
- Make FakeRenderWidgetHostViewAura only install delegate frame host
  when it is supported (i.e. RWHVAura has a valid |frame_sink_id_|);
- Early out a few of tests that do not work in mash;

And content_unittests still uses TestWindowTreeClientSetup since the
tests are happy with it and do not need test_ws.

Bug: 866942
Change-Id: I664470f0efba1f350a2ae22cb978e0f3f9b2d1f5
Reviewed-on: https://chromium-review.googlesource.com/1153768
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579836}
6 files changed