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This CL adds a reusable bubble and corresponding WebUIController
class to be used when implementing Secondary UI using WebUI.

See the below doc for more details.

https://docs.google.com/document/d/1UtAVhVpo8Si_XlZEdt-BH_AN4egBo1XppZEhF3_bthM/edit?usp=sharing

Bug: 1099917
Change-Id: I6b16997b05dc2bf2f208ca71dd5b3cfa11db00f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462223
Commit-Queue: Thomas Lukaszewicz <tluk@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817521}