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This change restructures URL bar and omnibox code in several ways:

- Change all buttons to have transparent backgrounds, such that the
  UX-specified hover colors look correct.
- Enable background color animation on ALL buttons.
- Simplify the omnibox into a layout of transparent-background elements,
  with a single backblane (one element with rounded corners, background
  color and shadow).
- Make better use of layouts to remove instances of anchoring.
- Introduce helper functions to translate UX spec colors to constants.

BUG=821946

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_vr;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I0dd6c9064cdec520ba45f0ad62aedf8fb4ad1364
Reviewed-on: https://chromium-review.googlesource.com/981050
Commit-Queue: Christopher Grant <cjgrant@chromium.org>
Reviewed-by: Ian Vollick <vollick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545957}