[Chromecast] Fix cast_media_unittests build

Currently cast_media_unittests can't build on platforms
(enable_video_with_mixed_audio == true && use_alsa == false), because of
not finding the VideoDecoderForMixer::InitializeGraphicsForTesting
symbol.

This symbol isn't useful on those platforms, so define a dummy that can be used in
this case.

BUG=b/124520050
TEST=Build cast_media_unittests

Change-Id: I35abfc3a549cd1fa185534d7b48d8dd93db14e04
Reviewed-on: https://chromium-review.googlesource.com/c/1475048
Commit-Queue: Mina Almasry <almasrymina@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Auto-Submit: Mina Almasry <almasrymina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633876}

chromecast/media/cma/BUILD.gn

@@ -77,7 +77,16 @@ cast_source_set("unittests") {
 
   if (enable_video_with_mixed_audio) {
     defines = [ "ENABLE_VIDEO_WITH_MIXED_AUDIO" ]
-    libs = ["videodecoderformixer"]
+
+    # libvideodecoderformixer is linked here for the
+    # VideoDecoderForMixer::InitializeGraphicsForTesting symbol. Currently only
+    # alsa platforms do anything useful in this initialization, so otherwise
+    # just link in a dummy.
+    if (use_alsa) {
+      libs = [ "videodecoderformixer" ]
+    } else {
+      sources += [ "backend/dummy_initialize_graphics_for_testing.cc" ]
+    }
   }
 
   data = [

chromecast/media/cma/backend/dummy_initialize_graphics_for_testing.cc

+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "chromecast/media/cma/backend/video_decoder_for_mixer.h"
+
+namespace chromecast {
+namespace media {
+
+CHROMECAST_EXPORT void VideoDecoderForMixer::InitializeGraphicsForTesting() {
+  // No initialization required.
+}
+
+}  // namespace media
+}  // namespace chromecast