Disable a flaky test on Windows.

Disables DownloadExtensionTest.DownloadExtensionTest_OnDeterminingFilename_ReservedFilename
on Windows.

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

Review URL: https://codereview.chromium.org/26293004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227959 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227959