[Chromecast] Remove |audio_device_id_| from CastRenderer

1. The |audio_device_id| in CastRenderer is always
::media::AudioDeviceDescription::kDefaultDeviceId, so it's
not needed.

2. Add CastMojoMediaClient::CreateCastRenderer() method, so when
CastRendererFactory in the upstream is implemented and used, the
CastRenderer can be created and used without breaking chromecast.

No behavior change is expected.

Bug: crbug.com/925450
Test: 1. Manually Cast. 2. Internal CQ.

Change-Id: Ia2234f721ba6b636ea87a67198894aeeba173bde
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1626389
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Guohui Deng <guohuideng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663282}
4 files changed