Disable flaky PolicyTest.FileURLBlacklist.

Recent flake:
http://build.chromium.org/p/chromium.win/builders/Win7%20Tests%20%28dbg%29%283%29/builds/24993

Flakiness dashboard:
http://chromium-build-logs.appspot.com/gtest_query?gtest_query=PolicyTest.FileURLBlacklist

BUG=339240
TEST=none
R=erg@chromium.org
TBR=joaodasilva@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/139413008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248574 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248574