[Chromecast] Set use_alsa=true only for Chromecast desktop & audio.

For Cast builds, only cast desktop and cast audio-only products should use_alsa=true
by default.

BUG= internal b/29770818
TEST= Build several Cast platforms

Review-Url: https://codereview.chromium.org/2109753002
Cr-Commit-Position: refs/heads/master@{#402674}
1 file changed