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This CL updates the bubble width to use |kTrayMenuWidth|. This CL also
includes minor refactoring such as:
- Replaces hard-coded constants with calculation.
- Introduces ui_constants.h to store both appearance and URL constants.

Screenshot: https://screenshot.googleplex.com/6PBiuFovTjyfjaa.png

BUG=1143058,1106937

Change-Id: Icdf17321a9acd9be53b7287023e197e027fcac62
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2510551
Commit-Queue: Meilin Wang <meilinw@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Tim Song <tengs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822893}