Make the uninstall dialog use the correct parent window on Athena

Use the associated contents instead of the extensions::WindowController to get the parent window for the "uninstall prompt" dialog.
This results in the correct parent window used for uninstall dialogs on Athena
because there is no extensions::WindowController for Athena.

BUG=414341
TEST=None

Committed: https://crrev.com/b3d3cef2bec5fd09245d0038bf5decdfaa500642
Cr-Commit-Position: refs/heads/master@{#300545}

[pkotwicz, 2014-10-21 16:05:55]

pkotwicz@chromium.org changed reviewers:
+ rockot@chromium.org

[pkotwicz, 2014-10-21 16:05:55]

rockot@ can you please take a look?

management.createAppShortcut is also problematic on Athena but I am unsure
whether we want to support it yet on Athena so I am leaving it as is.

The next tasks for me are:
- Figure out how to enable relevant api tests on Athena
- Continuing work on crbug.com/422814

[Ken Rockot(use gerrit already), 2014-10-21 18:31:14]

LGTM but please fix the CL description.

It's fine to make a note of why you made this change as well, but the
description should primarily serve to explain what the change is.

This changes the management uninstall functions to get a native window from
their WebContents rather than depending on the extensions window controller. The
fact that it prevents Athena from crashing is a useful side-effect.

[pkotwicz, 2014-10-21 18:59:20]

The CQ bit was checked by pkotwicz@chromium.org

[commit-bot: I haz the power, 2014-10-21 19:01:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/636083003/1

[commit-bot: I haz the power, 2014-10-21 20:13:12]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-21 20:15:20]

Patchset 1 (id:??) landed as
https://crrev.com/b3d3cef2bec5fd09245d0038bf5decdfaa500642
Cr-Commit-Position: refs/heads/master@{#300545}