Disable DownloadExtensionTest_OnDeterminingFilename_NoChange

This test appears to be failing intermittently on the waterfall.

BUG=302071
TBR=benjhayden@chromium.org

Review URL: https://codereview.chromium.org/25398002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226083 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226083