Fix ESC not closing the print preview dialog on Mac

Fix ESC not closing the print preview dialog on Mac

Making toolkit-views available for non-browser UI means that this guard
in print_preview.js needs updating to ensure ESC still closes the dialog
on Mac. The webui container is still Cocoa.

Everything is toolkit-views now (except mobile/cast), so just check for
mac.

BUG=469045
Committed: https://crrev.com/c29919ce9a98620c21e6e7750a3bb7ed2afebbbe
Cr-Commit-Position: refs/heads/master@{#321607}

[tapted, 2015-03-20 10:15:19]

tapted@chromium.org changed reviewers:
+ alekseys@chromium.org

[tapted, 2015-03-20 10:18:10]

Hi Aleksey, please take a look

[Aleksey Shlyapnikov, 2015-03-20 17:47:24]

lgtm

[Andre, 2015-03-20 18:41:32]

The CQ bit was checked by andresantoso@chromium.org

[commit-bot: I haz the power, 2015-03-20 18:42:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1025613002/20001

[commit-bot: I haz the power, 2015-03-20 18:48:57]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-03-20 18:49:49]

Patchset 2 (id:??) landed as
https://crrev.com/c29919ce9a98620c21e6e7750a3bb7ed2afebbbe
Cr-Commit-Position: refs/heads/master@{#321607}