Disable BackgroundContentsServiceNotificationTest for Linux-Gtk.

My previous CL enabled them for Linux, but my intention was
to enable for Linux-Aura.

BUG=None
TEST=unit_tests
TBR=atwilson@chromium.org

Review URL: https://codereview.chromium.org/183853007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254190 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254190