message: Fix build with rtc_libvpx_build_vp9=false
  Like aom and openh264, VP9 can be disabled with the gn argument.
  Cr-Commit-Position: refs/heads/master@{#33737}
diff: @@ -858,9 +858,12 @@ if (rtc_include_tests) {
  "codecs/test/video_encoder_decoder_instantiation_tests.cc",
  "codecs/test/videocodec_test_libvpx.cc",
  "codecs/vp8/test/vp8_impl_unittest.cc",
  - "codecs/vp9/test/vp9_impl_unittest.cc",
  ]
  + if (rtc_libvpx_build_vp9) {
  + sources += [ "codecs/vp9/test/vp9_impl_unittest.cc" ]
  + }
  +
  # TODO(jianj): Fix crash on iOS and re-enable
  if (enable_libaom && !is_ios) {
  sources += [ "codecs/test/videocodec_test_libaom.cc" ]
  