- Disable CSS transition in the layout test mode.
- Disable auto-hiding behavior in the layout test mode.
- Enable this feature as "experimental"
  ValidationMessage request dump in test_runner isn't used any longer.
  This CL needs to update some -expected.txt of existing tests.
- Add two pixel tests.

Bug: 739091
Change-Id: I33285b345af4d76e2cf6ebf6d5ee0707c14792d8
Reviewed-on: https://chromium-review.googlesource.com/560919Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#484478}