GitLab

This CL implements a volume slider to UnifiedSystemTray.

This CL adds following classes:
* UnifiedVolumeView
  * View class of a volume slider that inherits UnifiedSliderView.
    It observes CrasAudioHandler for current volume and mute state.
    User can toggle mute state by pressing the button.
* UnifiedVolumeSliderController
  * Controller class of the volume slider.
* UnifiedSliderButton
  * Added to support toggling. Inherits TopShortcutButton.

Screenshot: http://screen/AAhfnLdBdHH

UnifiedSystemTray design doc: go/cros-qs-restyling

TEST=manual
BUG=819943

Change-Id: I1af89386f3e5865491f052bbb323f4431c26880a
Reviewed-on: https://chromium-review.googlesource.com/958886Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543290}