[Extensions Toolbar Mac] Fix crash in browser action context menus

[Extensions Toolbar Mac] Fix crash in browser action context menus

Right now, there's a chance that the NSMenu can outlive the underlying menu
model (which is owned by the view controller). Make sure we reset the context
menu before destruction and before creating a new menu to correct this.

BUG=462175
Committed: https://crrev.com/32f8b1c3d6a0e0f967c27ecd1754efed849106c6
Cr-Commit-Position: refs/heads/master@{#319678}

[Devlin, 2015-03-05 19:23:06]

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

[Devlin, 2015-03-05 19:23:07]

Avi, mind taking a quick look?

[Avi (use Gerrit), 2015-03-05 23:09:29]

lgtm

[Devlin, 2015-03-09 16:31:06]

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

[commit-bot: I haz the power, 2015-03-09 16:31:37]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/967883006/1

[commit-bot: I haz the power, 2015-03-09 17:52:20]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-09 17:53:33]

Patchset 1 (id:??) landed as
https://crrev.com/32f8b1c3d6a0e0f967c27ecd1754efed849106c6
Cr-Commit-Position: refs/heads/master@{#319678}