Revert of Revert of Introduce GetBackendFromContextId to avoid using list index.

Revert of Revert of Introduce GetBackendFromContextId to avoid using list index. (patchset #1 id:1 of https://codereview.chromium.org/573573003/)

Reason for revert:
Revert the revert since the bot is still red after the revert.

Original issue's description:
> 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}

TBR=tbarzic@chromium.org,achuith@chromium.org,tonyg@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=414267
Committed: https://crrev.com/872bf7868e04a6354f20d66d069cb623b1a70298
Cr-Commit-Position: refs/heads/master@{#295205}

[nednguyen, 2014-09-17 01:10:51]

Created Revert of Revert of Introduce GetBackendFromContextId to avoid using
list index.

[commit-bot: I haz the power, 2014-09-17 01:13:39]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/578693002/1

[commit-bot: I haz the power, 2014-09-17 01:14:24]

Committed patchset #1 (id:1) as d10989a5b63f2f29618ccdbad310e409cb97cc96

[commit-bot: I haz the power, 2014-09-17 01:14:54]

Patchset 1 (id:??) landed as
https://crrev.com/872bf7868e04a6354f20d66d069cb623b1a70298
Cr-Commit-Position: refs/heads/master@{#295205}