[Chromecast] Unittests should not use null video decoder if video is enabled.

If enable_video_with_mixed_audio is defined, unittests should not
include null_video source_set.

Bug: 80443515
Test: manual / CQ
Change-Id: I905edbdc019841f1d2dfe18169ef3ba0af92b270
Reviewed-on: https://chromium-review.googlesource.com/1220697Reviewed-by: Sergey Volk <servolk@chromium.org>
Commit-Queue: Sunil Kumar <skx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590538}

chromecast/media/cma/BUILD.gn

@@ -36,13 +36,16 @@ cast_source_set("test_support") {
     "//base",
     "//chromecast/common/mojom",
     "//chromecast/media/cma/backend",
-    "//chromecast/media/cma/backend:null_video",
     "//chromecast/media/cma/base",
     "//media:test_support",
     "//testing/gmock",
     "//testing/gtest",
     "//ui/gfx/geometry",
   ]
+
+  if (!enable_video_with_mixed_audio) {
+    deps += [ "//chromecast/media/cma/backend:null_video" ]
+  }
 }
 
 cast_source_set("unittests") {