aw: Prevent multiple AddObserver

aw: Prevent multiple AddObserver

StreamTextureFactorySynchronousImpl::CreateProxy did not check if
context_provider already exists in which case it does not need to re-add
the observer.

BUG=416981
Committed: https://crrev.com/da857ef3f2e03ab5aa792d0d29d97c83ac50a6b1
Cr-Commit-Position: refs/heads/master@{#296303}

[boliu, 2014-09-23 20:52:34]

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

[boliu, 2014-09-23 20:52:35]

PTAL. Tiny fix

[qinmin, 2014-09-23 21:57:31]

lgtm

[boliu, 2014-09-23 22:04:14]

The CQ bit was checked by boliu@chromium.org

[commit-bot: I haz the power, 2014-09-23 22:05:45]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/594273003/1

[commit-bot: I haz the power, 2014-09-23 23:44:55]

Committed patchset #1 (id:1) as 2d229ae5f966ebd9b942e169b0361c8e322ea73e

[commit-bot: I haz the power, 2014-09-23 23:45:45]

Patchset 1 (id:??) landed as
https://crrev.com/da857ef3f2e03ab5aa792d0d29d97c83ac50a6b1
Cr-Commit-Position: refs/heads/master@{#296303}