[Mac] Fix accessibility crash because I am terrible with dynamic types.

[Mac] Fix accessibility crash because I am terrible with dynamic types.

Turns out attributes like AXPosition will crash with @[] :/

BUG=412707
TEST=Start Chrome with --enable-new-avatar-menu. Provided you have zooming
enabled in your Mac Accessibility settings, press Ctrl and scroll up to zoom
in. Click on the avatar button. The avatar bubble should open and Chrome should
not crash.

Committed: https://crrev.com/cc93e76168da60dbae624dcdb49da33ea0944b68
Cr-Commit-Position: refs/heads/master@{#299160}

[noms (inactive), 2014-10-10 15:00:28]

noms@chromium.org changed reviewers:
+ groby@chromium.org

[groby-ooo-7-16, 2014-10-10 18:24:53]

Gah. LGTM, sorry for not catching it in the first place.

[noms (inactive), 2014-10-10 18:26:09]

The CQ bit was checked by noms@chromium.org

[commit-bot: I haz the power, 2014-10-10 18:27:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/645933002/1

[commit-bot: I haz the power, 2014-10-10 19:23:17]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-10 19:24:02]

Patchset 1 (id:??) landed as
https://crrev.com/cc93e76168da60dbae624dcdb49da33ea0944b68
Cr-Commit-Position: refs/heads/master@{#299160}