Add cast manifest overlay for media service

BUG=b:110480251
TEST=Build

Change-Id: I5ddc40645ad60d32be0fb1a504c4f0615dc505ea
Reviewed-on: https://chromium-review.googlesource.com/1113813Reviewed-by: Ken Rockot <rockot@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Aidan Wolter <awolter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571772}

media/mojo/services/media_manifest.json

@@ -7,7 +7,8 @@
         "media:media": [ "media.mojom.MediaService" ]
       },
       "requires": {
-        "*": [ "app" ]
+        "*": [ "app" ],
+        "chromecast": [ "multizone" ]
       }
     }
   }