[Extensions Toolbar Mac] Allow the overflow toolbar to use the theme provider

[Extensions Toolbar Mac] Allow the overflow toolbar to use the theme provider

The overflow menu icons previously didn't have any images because there isn't
a theme provider for the menu's window. Allow image cells to provide a different
theme provider so these can display properly.

BUG=429810
Committed: https://crrev.com/72b4a2c50bf2bc01751aa5a98494bd943955a216
Cr-Commit-Position: refs/heads/master@{#308401}

[Devlin, 2014-12-15 18:14:09]

Patchset #1 (id:1) has been deleted

[Devlin, 2014-12-15 18:14:14]

Patchset #1 (id:20001) has been deleted

[Devlin, 2014-12-15 19:32:22]

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

[Devlin, 2014-12-15 19:32:22]

Avi, please take a look when you can.

[Avi (use Gerrit), 2014-12-15 19:41:51]

lgtm

[Devlin, 2014-12-15 19:42:16]

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

[commit-bot: I haz the power, 2014-12-15 19:43:33]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/803613002/40001

[commit-bot: I haz the power, 2014-12-15 19:59:59]

Committed patchset #1 (id:40001)

[commit-bot: I haz the power, 2014-12-15 20:01:07]

Patchset 1 (id:??) landed as
https://crrev.com/72b4a2c50bf2bc01751aa5a98494bd943955a216
Cr-Commit-Position: refs/heads/master@{#308401}