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- Add CastDialogSinkButton, which will be used for sink list items in
  the Harmony Cast dialog. It's a subclass of HoverButton that shows a
  device icon and an optional status indicator (checkmark or throbber).
- Add UIMediaSink, a struct containing data used to populate the button
- Add Skia icons used by the button: Cast for EDU, input, speaker,
  speaker group, and TV

Screenshot of the button (two of them) in a WIP dialog:
https://drive.google.com/file/d/1M1R9jO-VHEO18aA3pTpoQBI4FHMU5MEf/view?usp=sharing

Dimensions and colors are subject to change in later CLs.

Bug: 826089, 826393
Change-Id: I0eef58e1c0b9bf2cff3b3d146e911d3a18c7d23c
Reviewed-on: https://chromium-review.googlesource.com/996458
Commit-Queue: Takumi Fujimoto <takumif@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Adam Parker <amp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550722}