Allow audio requests in getDisplayMedia() calls from blink

This CL adds cases to allow for getDisplayMedia({video:true, audio:true}) calls.
- Remove the earlier checks in blink that adds exceptions for audio requests.
- Reject audio-only requests.
- Set up UserMediaRequest appropriately and allow for settings in
MediaStreamManager.
- Add a FakeUI case to create a dummy AudioDevice.
- Add unit tests and layout tests.

Bug: 896333
Change-Id: I627c8ce84d752528430c4d46c0c5f472b39706d1
Reviewed-on: https://chromium-review.googlesource.com/c/1444353
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Emircan Uysaler <emircan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628366}
7 files changed