[chromecast] Disable WebM, Vorbis tests for ALSA

The ALSA-based CMA backend doesn't support that functionality yet.

BUG=internal b/19544200

Review URL: https://codereview.chromium.org/1175843002

Cr-Commit-Position: refs/heads/master@{#333766}

chromecast/chromecast_tests.gypi

@@ -122,6 +122,14 @@
             ],
           },
         }],
+        ['disable_display==1', {
+          'variables': {
+            'filters': [
+              # These are not supported by the backend right now. b/21737919
+              'cast_media_unittests --gtest_filter=-AudioVideoPipelineDeviceTest.VorbisPlayback:AudioVideoPipelineDeviceTest.WebmPlayback',
+            ],
+          }
+        }],
       ],
       'includes': ['build/tests/test_list.gypi'],
     },