Don't return wall clock times when time isn't ticking.

Don't return wall clock times when time isn't ticking.

The video renderer may request wall clock times before the audio
renderer has notified the renderer that it has underflowed.  We
should refuse to convert wall clock times as soon as an underflow
state is detected, instead of waiting for the renderer to pause
everything on the media thread (which may be backed up for decoding
or other events).

BUG=489503
TEST=modified underflow test.

Committed: https://crrev.com/c49e7c5523df7164ddada93fb8d826e96d0752f3
Cr-Commit-Position: refs/heads/master@{#330481}

[DaleCurtis, 2015-05-19 00:36:16]

dalecurtis@chromium.org changed reviewers:
+ xhwang@chromium.org

[xhwang, 2015-05-19 03:03:25]

lgtm

[DaleCurtis, 2015-05-19 04:20:46]

The CQ bit was checked by dalecurtis@chromium.org

[commit-bot: I haz the power, 2015-05-19 04:21:05]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1137323005/20001

[commit-bot: I haz the power, 2015-05-19 05:33:37]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-05-19 05:34:40]

Patchset 2 (id:??) landed as
https://crrev.com/c49e7c5523df7164ddada93fb8d826e96d0752f3
Cr-Commit-Position: refs/heads/master@{#330481}