Fix potential deadlock between Pipeline and VideoRendererImpl.

Fix potential deadlock between Pipeline and VideoRendererImpl.

I don't believe we hit this in practice since the two lock acquiring
sequences we make happen as separate tasks executed on the same
thread.

To summarize the ThreadSanitizer warning:
  - Task A acquires VideoRendererImpl -> Pipeline lock via
    FrameReady() -> GetMediaDuration()
  - Task B acquires Pipeline -> VideoRendererImpl lock via
    DoStop() -> ~VideoRendererImpl()

Solution: don't hold onto the Pipeline lock when destroying renderers. 

BUG=412097
Committed: https://crrev.com/0abb84994f293d6fc17e5c2d3e21a933d4f73fbc
Cr-Commit-Position: refs/heads/master@{#293890}

[scherkus (not reviewing), 2014-09-09 00:51:09]

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

[xhwang, 2014-09-09 00:57:24]

lgtm!

[scherkus (not reviewing), 2014-09-09 00:59:00]

The CQ bit was checked by scherkus@chromium.org

[commit-bot: I haz the power, 2014-09-09 01:09:46]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/scherkus@chromium.org/557603002/1

[commit-bot: I haz the power, 2014-09-09 07:43:22]

Committed patchset #1 (id:1) as abe0f231fbeb317e2fe19ae81db70014d2a6f86c

[commit-bot: I haz the power, 2014-09-10 03:51:48]

Patchset 1 (id:??) landed as
https://crrev.com/0abb84994f293d6fc17e5c2d3e21a933d4f73fbc
Cr-Commit-Position: refs/heads/master@{#293890}