Pass camera facing info when camera is started to audio client
so that active audio device can be switched according to camera
facing.

BUG=672695
TEST=Print log in CrasAudioClientImpl::OnVideoCaptureStarted()
and verify that facing info is correctly passed.

Review-Url: https://codereview.chromium.org/2634263002
Cr-Commit-Position: refs/heads/master@{#452180}