Disable audio tests for chromecast devices

cr-merge is blocked:
https://eureka-internal-review.git.corp.google.com/c/chromium/src/+/179137

GN target "for_mixer_audio" introduces un-implemented interface to
chromecast devices, which breaks the build.

Bug: 110113636
Test: local build
Change-Id: I13f73eead3bbd889ef3fe10714951ce1e9a20008
Reviewed-on: https://chromium-review.googlesource.com/1100173Reviewed-by: Simeon Anfinrud <sanfin@chromium.org>
Reviewed-by: Stephen Lanham <slan@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Linkun Chen <lkchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567400}

chromecast/BUILD.gn

@@ -96,7 +96,7 @@ cast_test_group("cast_tests") {
     ]
   }
 
-  if (is_linux || is_fuchsia) {
+  if ((is_linux || is_fuchsia) && (use_alsa || enable_video_with_mixed_audio)) {
     tests += [
       "//chromecast/media/cma/backend:cast_audio_backend_unittests",
       "//chromecast/media/cma/backend:cast_av_sync_backend_unittests",