Chromecast: adds fake libffmpegsumo target for more graceful internal build.

R=alokp@chromium.org
BUG=b/21639416

Review URL: https://codereview.chromium.org/1165943004

Cr-Commit-Position: refs/heads/master@{#332943}

chromecast/chromecast.gyp

@@ -51,10 +51,6 @@
         'public/video_plane.h',
       ],
     },
-    # TODO(gunsch): Remove this fake target once it's either added or no
-    # longer referenced from internal code.
-    {'target_name': 'cast_media_audio', 'type': 'none'},
-
     {
       'target_name': 'cast_base',
       'type': '<(component)',

chromecast/media/media.gyp

@@ -10,6 +10,13 @@
     'use_default_libcast_media%': 1,
   },
   'targets': [
+    # TODO(gunsch): delete this target once Chromecast M44/earlier is obsolete.
+    # See: b/21639416
+    {
+      'target_name': 'libffmpegsumo',
+      'type': 'loadable_module',
+      'sources': ['empty.cc'],
+    },
     {
       'target_name': 'media_base',
       'type': '<(component)',