Add MEDIA_DISPLAY_AUDIO_CAPTURE enum for getDisplayMedia with audio

This CL adds a new enum to handle audio capture requests originating from
getDisplayMedia(). Also adds the corresponding mojom enum for passing it between
renderer and browser processes.

See https://chromium-review.googlesource.com/c/chromium/src/+/1432752 for a
playground CL where audio capture is hooked through.

Bug: 896333
Change-Id: I197995a255961f4523a360629efc239539a1fc34
Reviewed-on: https://chromium-review.googlesource.com/c/1437520
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Weiyong Yao <braveyao@chromium.org>
Commit-Queue: Emircan Uysaler <emircan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626734}
6 files changed