Cast: Re-enable tests runs on windows

These tests were failing becuase the media library had not been initialized.
This has now been fixed and the tests can be re-enabled.

BUG=329563

Review URL: https://codereview.chromium.org/178983003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253903 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253903