Introduce GetBackendFromContextId to avoid using list index.

Introduce GetBackendFromContextId to avoid using list index.

BUG=398467
TEST=ExtensionTest

Committed: https://crrev.com/a1a83005087fdac70e7809f9dc1d89f6c4f88dfc
Cr-Commit-Position: refs/heads/master@{#294455}

[achuithb, 2014-09-11 11:49:53]

achuith@chromium.org changed reviewers:
+ nednguyen@google.com

[achuithb, 2014-09-11 11:49:53]

Updated your CL with the changes to AllExtensionsLoaded()

[achuithb, 2014-09-11 11:52:39]

achuith@chromium.org changed reviewers:
+ tbarzic@chromium.org

[nednguyen, 2014-09-11 13:45:22]

On 2014/09/11 11:52:40, achuithb wrote:

Lgtm

[achuithb, 2014-09-11 13:54:49]

On 2014/09/11 13:45:22, nednguyen wrote:
> On 2014/09/11 11:52:40, achuithb wrote:
> 
> Lgtm

ty

[achuithb, 2014-09-11 13:54:53]

The CQ bit was checked by achuith@chromium.org

[commit-bot: I haz the power, 2014-09-11 13:55:54]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/556003005/1

[commit-bot: I haz the power, 2014-09-11 19:57:59]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-09-11 19:58:02]

Exceeded time limit waiting for builds to trigger.

[nednguyen, 2014-09-11 20:02:45]

The CQ bit was checked by nednguyen@google.com

[commit-bot: I haz the power, 2014-09-11 20:07:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/556003005/1

[commit-bot: I haz the power, 2014-09-11 20:23:49]

Committed patchset #1 (id:1) as 6d02af6f42731c3ac203dbff39e1df07590e7155

[commit-bot: I haz the power, 2014-09-11 20:35:06]

Patchset 1 (id:??) landed as
https://crrev.com/a1a83005087fdac70e7809f9dc1d89f6c4f88dfc
Cr-Commit-Position: refs/heads/master@{#294455}

[nednguyen, 2014-09-16 01:29:44]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/573573003/ by nednguyen@google.com.

The reason for reverting is: It looks like the moto E perf bots are broken due
to this change.
.