Disable PanelWithTransientChild test for Win8

This got broken when the test was added. The test needs to be disabled for any platform that does not support resizable root window.

BUG=304544
TEST=ash_unittests --gtest_filter=*PanelWithTransientChild*

Review URL: https://codereview.chromium.org/26264002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227312 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227312