[Chromecast] Wait until the real backend is initialized to call SetVolume

Bug: internal b/120519408
Change-Id: I64eba756d8d8bc10c867bd354c0e9823e788ad07
Reviewed-on: https://chromium-review.googlesource.com/c/1388714
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Kenneth MacKay <kmackay@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623775}
3 files changed