Cast Extensions API: Get rid of unnecessary callbacks

Some getter and utility methods don't need a callback to return the
results. Results can simply be returned.

Updated test to verify caps and parameters are populated correctly.

BUG=301920

Review URL: https://codereview.chromium.org/81993002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236730 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236730