Disable DownloadExtensionTests

R=grt@chromium.org
TBR=benjhayden@chromium.org
NOTRY=true
BUG=306144

Review URL: https://codereview.chromium.org/26501006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228255 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228255