Fix cast_unittests on linux_chromeos

cast_unittests doesn't seem to build on linux_chromeos this make sure
there's such target on all platforms.

TBR=mikhal@chromium.org
BUG=329563

Review URL: https://codereview.chromium.org/108193018

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241785 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241785