[Extensions] Cache extension action icons

[Extensions] Cache extension action icons

Cache extension action icons so that they don't have to be
reloaded each time a new view is created (i.e., each time a
new browser or the new overflow menu is opened).

BUG=452958
Committed: https://crrev.com/f19c7663b819419271996f47be3f2be55db95fb7
Cr-Commit-Position: refs/heads/master@{#313724}

[Devlin, 2015-01-28 20:56:13]

Patchset #1 (id:1) has been deleted

[Devlin, 2015-01-28 20:56:18]

Patchset #1 (id:20001) has been deleted

[Devlin, 2015-01-28 20:57:58]

rdevlin.cronin@chromium.org changed reviewers:
+ finnur@chromium.org

[Devlin, 2015-01-28 20:57:59]

Please take a look when you can :)

[Finnur, 2015-01-29 09:26:45]

LGTM

[Devlin, 2015-01-29 16:54:08]

The CQ bit was checked by rdevlin.cronin@chromium.org

[commit-bot: I haz the power, 2015-01-29 16:54:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/885443004/60001

[commit-bot: I haz the power, 2015-01-29 16:58:02]

Committed patchset #2 (id:60001)

[commit-bot: I haz the power, 2015-01-29 16:59:10]

Patchset 2 (id:??) landed as
https://crrev.com/f19c7663b819419271996f47be3f2be55db95fb7
Cr-Commit-Position: refs/heads/master@{#313724}