Disable flaky ExtensionInstallBlacklistWildcard browser test.

Failing on WinXP: http://build.chromium.org/p/chromium.win/builders/XP%20Tests%20%28dbg%29%283%29/builds/40435
Flakiness dashboard: http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Chromium&testType=browser_tests&tests=PolicyTest.ExtensionInstallBlacklistWildcard

BUG=307994
TBR=joaodasilva

Review URL: https://codereview.chromium.org/27056007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228909 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228909