Preroll decoders when video starts playing

Preroll decoders when video starts playing

When video starts playing, HTMLMediaElement no longer issues a seek(0) call.
As a result, decoders needs to be prerolled so that video and audio will start outputting frames at the same time.
This change also fixes an issue that when EOS is reached, the decoder does not clean up the prerolling state

internal b/17812723

BUG=

Committed: https://crrev.com/bbc157765fb43670db2f14deae6a6b3e9d5e91c1
Cr-Commit-Position: refs/heads/master@{#299006}

[qinmin, 2014-10-08 22:39:07]

qinmin@chromium.org changed reviewers:
+ wolenetz@chromium.org

[qinmin, 2014-10-08 22:39:07]

PTAL

[wolenetz, 2014-10-09 21:01:08]

lgtm

[qinmin, 2014-10-09 21:53:58]

The CQ bit was checked by qinmin@chromium.org

[commit-bot: I haz the power, 2014-10-09 21:54:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/638233002/1

[commit-bot: I haz the power, 2014-10-09 23:18:33]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-09 23:19:33]

Patchset 1 (id:??) landed as
https://crrev.com/bbc157765fb43670db2f14deae6a6b3e9d5e91c1
Cr-Commit-Position: refs/heads/master@{#299006}