Fixes _NET_ACTIVE_WINDOW support check in order to not always return no.

The check is currently not working on Unity because it expects an array
of atoms instead of one XID. With this patch, Unity is correctly seen
as supporting the feature.

BUG=None

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=246270

Review URL: https://codereview.chromium.org/138903007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246520 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246520