Fix cast.gyp

Define 'chromium_code' : 1 in media/cast/cast.gyp.

Review URL: https://codereview.chromium.org/37003009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230595 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230595