Add autotest API to check WS client output

- Add an ensureWindowServiceClientHasDrawnWindow method to check
  a Window Service client really paints on screen.
- Add a chrome_content_browser_manifest_test_overlay.json that is
  only applied with --use-test-config switch so that the autotest
  API could make a mojo call to WindowServerTest.

Bug: 908545
Change-Id: I72f9b64fa06b4acf8e7fb8440dd65375949ef3c7
Reviewed-on: https://chromium-review.googlesource.com/c/1406155
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622088}
9 files changed