Revert 230925 "Speculatively disabling ExtensionServiceTest.SetU..."

The bot continued to fail after disabling the test.

> Speculatively disabling ExtensionServiceTest.SetUnsetBlacklistInPrefs
> 
> chromium.webkit Vista Tests keeps failing on it:
> http://build.chromium.org/p/chromium.webkit/builders/Vista%20Tests/builds/34132/
> 
> This bot has had issues for the past two weeks. It may be a deeper problem.
> 
> BUG=310941
> TBR=jsbell
> 
> Review URL: https://codereview.chromium.org/43053002

TBR=falken@chromium.org

Review URL: https://codereview.chromium.org/43303002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230966 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230966