[Chromecast] Enable MultizoneBackendTest on Earth.

chromecast/media/cma/BUILD.gn

Index: chromecast/media/cma/BUILD.gn
diff --git a/chromecast/media/cma/BUILD.gn b/chromecast/media/cma/BUILD.gn
index 2ba1312f49d460b2f0aca61f93666434c6166726..dbfb5d9027b23c7ae4a7af616bc6c7c0e29dd312 100644
--- a/chromecast/media/cma/BUILD.gn
+++ b/chromecast/media/cma/BUILD.gn
@@ -74,9 +74,7 @@ source_set("unittests") {
   ]
 
   # MultizoneBackendTest verifies rendering delay reported by the backend.
-  # Since rendering delay is optional on video platforms, enable this test
-  # on audio devices only.
-  if (is_cast_audio_only && !is_cast_desktop_build) {
+  if (supports_multizone) {
     sources += [ "backend/multizone_backend_unittest.cc" ]
   }
 }