Enable WebrtcCastApiTest.Basics

Enable the test now that the problem with build bot is resolved.

BUG=314971

Review URL: https://codereview.chromium.org/61103002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233179 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233179