Revert of Introduce GetBackendFromContextId to avoid using list index.

Revert of Introduce GetBackendFromContextId to avoid using list index. (patchset #1 id:1 of https://codereview.chromium.org/556003005/)

Reason for revert:
It looks like the moto E perf bots are broken due to this change.

BUG=414267
Original issue's description:
> Introduce GetBackendFromContextId to avoid using list index.
>
> BUG=398467
> TEST=ExtensionTest
>
> Committed: https://crrev.com/a1a83005087fdac70e7809f9dc1d89f6c4f88dfc
> Cr-Commit-Position: refs/heads/master@{#294455}

TBR=tbarzic@chromium.org,achuith@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=398467
Committed: https://crrev.com/c8fad04c1e2119f9e8efc68f32d5268161d044d3
Cr-Commit-Position: refs/heads/master@{#295136}

[nednguyen, 2014-09-16 01:29:45]

Created Revert of Introduce GetBackendFromContextId to avoid using list index.

[nednguyen, 2014-09-16 01:30:09]

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

[nednguyen, 2014-09-16 01:30:13]

The CQ bit was checked by nednguyen@google.com

[nednguyen, 2014-09-16 01:30:47]

The CQ bit was unchecked by nednguyen@google.com

[achuithb, 2014-09-16 20:05:15]

On 2014/09/16 01:29:45, nednguyen wrote:
> Created Revert of Introduce GetBackendFromContextId to avoid using list index.

Has the revert been landed? I see this issue is closed.

lgtm

[nednguyen, 2014-09-16 20:19:55]

The CQ bit was checked by nednguyen@google.com

[commit-bot: I haz the power, 2014-09-16 20:21:09]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/573573003/1

[commit-bot: I haz the power, 2014-09-16 20:22:13]

Committed patchset #1 (id:1) as a37b3543311584951f98f2ca710ba4f9116e17fb

[commit-bot: I haz the power, 2014-09-16 20:22:41]

Patchset 1 (id:??) landed as
https://crrev.com/c8fad04c1e2119f9e8efc68f32d5268161d044d3
Cr-Commit-Position: refs/heads/master@{#295136}

[nednguyen, 2014-09-17 01:10:50]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/578693002/ by nednguyen@google.com.

The reason for reverting is: Revert the revert since the bot is still red after
the revert..