fuchsia: Remove unnecessary dependencies in cast_media_shlib_common, add dep on null_video

stream_mixer.h was causing a dep on base, but just remove that include
because it's no longer needed.

null_video dependency required to let libcast_media_1.0_avsync link.

Bug: 808287
Change-Id: If86449b7ebb394907b41820c21d1cc805569c717
Reviewed-on: https://chromium-review.googlesource.com/c/1298633Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602546}

chromecast/media/cma/backend/video/BUILD.gn

@@ -37,8 +37,6 @@ cast_source_set("cast_media_shlib_common") {
     "cast_media_shlib_common.cc",
   ]
   deps = [
-    "//chromecast/media/cma/backend:for_mixer_audio",
-    "//chromecast/public",
     "//chromecast/public/media",
   ]
 }
@@ -72,4 +70,8 @@ cast_shared_library("libcast_media_1.0_avsync") {
     deps += [ "//chromecast/media/cma/backend/alsa:cma_backend_support" ]
     libs = [ "videodecoderformixer" ]
   }
+
+  if (is_fuchsia) {
+    deps += [ "//chromecast/media/cma/backend:null_video" ]
+  }
 }

chromecast/media/cma/backend/video/cast_media_shlib_common.cc

@@ -2,9 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "chromecast/public/cast_media_shlib.h"
-
-#include "chromecast/media/cma/backend/stream_mixer.h"
 #include "chromecast/public/media/media_capabilities_shlib.h"
 
 namespace chromecast {