Increase default buffer size when back end can't recommend one.

Increase default buffer size when back end can't recommend one.
This should help avoid stuttering on older XP machines, or vista machines
where hardware sample rate != application sample rate.  The downside is
audio latency will be higher.
Old value was for 30ms buffers.
New value is for 50ms buffers.
TEST=verify w/ nhu...@adobe.com once in canary
BUG=http://code.google.com/p/chromium/issues/detail?id=133393

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=144258

[brettw, 2012-06-26 03:58:01]

lgtm

[commit-bot: I haz the power, 2012-06-26 19:23:59]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/nfullagar@google.com/10656027/6001

[commit-bot: I haz the power, 2012-06-26 20:29:18]

Change committed as 144258