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The motivation here is to introduce the volume slider tray UI into
Windows Ash.

This change does not actually add tray audio UI to other platforms,
only moves some non-chromeos specific things out of the chromeos
platform folders, so that we can hook in the native Audio APIs
of other platforms to the tray UI.

At a high level, this change does the following:

1) Move tray_audio out of chromeos/ so that other platforms can
use the UI that it provides.
2) Decouples TrayAudio from CrasAudioHandler by introducing a
new delegate, in a similar vein to what is done with all the other
handlers (e.g. the BluetoothHandler)
3) Move the additional views (VolumeView, etc) to their own files
so they can be used by other platforms as well.

TEST=
 On CrOS physical device, verified that:
 1) Keyboard volume buttons function appropriately and display the
    volume view
 2) AudioDetailView still is accessible at the correct times
 3) Adjusting the volume (via the keyboard shortcuts or via the
    slider) correctly change the output volume.
 4) UI still behaves correctly (e.g. icons update as the volume
    changes, mute button works, etc).

BUG=227247

Review URL: https://codereview.chromium.org/163953007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251959 0039d316-1c4b-4281-b951-d872f2087c98