[Chromecast] Move audio logging from PushFrame to Start.

Putting logging in CastAudioOutputStream::PushFrame was generating many
unnecessary tasks. Moving the logging call to Start() greatly reduces
the number of posted tasks and still records the metric properly.

BUG=internal b/23214538

Review URL: https://codereview.chromium.org/1365173005

Cr-Commit-Position: refs/heads/master@{#350867}
1 file changed