Clean up tests for chrome.app.window alwaysOnTop property

Removed TODOs in browser tests to check the result of setAlwaysOnTop().
Since this runs synchronously in the browser, just send a dummy async
request afterward and then query isAlwaysOnTop() in the callback.

BUG=None
TEST=browser_tests (PlatformAppBrowserTest.WindowsApiAlwaysOnTop*)

Review URL: https://codereview.chromium.org/103223006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239690 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239690