cros: Disable AppListStartPageWebUITest

BUG=339340
TBR=xiyuan@chromium.org

Review URL: https://codereview.chromium.org/149173013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248054 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248054