Remove vpx video unit test from cast audio build.

Avoid run video only test on audio only build.

BUG=internal b/65382374
TEST=Build

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Idcfa3fa2cc3d0e3e54a311274bcb05fda1507c58
Reviewed-on: https://chromium-review.googlesource.com/673128Reviewed-by: Frank Liberato <liberato@chromium.org>
Reviewed-by: Sergey Volk <servolk@chromium.org>
Commit-Queue: Yuchen Liu <yucliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502944}

media/filters/BUILD.gn

@@ -305,7 +305,7 @@ source_set("unit_tests") {
     }
   }
 
-  if (media_use_libvpx) {
+  if (media_use_libvpx && !is_cast_audio_only) {
     sources += [ "vpx_video_decoder_unittest.cc" ]
   }