[Extensions Toolbar Mac] Implement drag-and-drop for the overflow menu

[Extensions Toolbar Mac] Implement drag-and-drop for the overflow menu

Make the overflow menu for the extensions toolbar on mac work with drag-and-
drop across multiple menu rows.
Also adjust the visibility criteria for the overflow menu in the wrench menu model.

TBR=sky@chromium.org (small wrench menu model change)

BUG=429810
Committed: https://crrev.com/1337ccff19b0a3fb7a5b0b09aea345c911279cfe
Cr-Commit-Position: refs/heads/master@{#309462}

[Devlin, 2014-12-22 19:11:59]

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

[Avi (use Gerrit), 2014-12-22 19:14:07]

lgtm

[Devlin, 2014-12-22 19:16:35]

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

[Devlin, 2014-12-22 20:48:42]

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

[commit-bot: I haz the power, 2014-12-22 20:49:19]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/820713008/1

[commit-bot: I haz the power, 2014-12-22 21:04:36]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-12-22 21:05:13]

Patchset 1 (id:??) landed as
https://crrev.com/1337ccff19b0a3fb7a5b0b09aea345c911279cfe
Cr-Commit-Position: refs/heads/master@{#309462}