Enable 'gn check' for //media/cast

.gn

 # This file is used by the GN meta build system to find the root of the source
 # tree and to set startup options. For documentation on the values set in this
 # file, run "gn help dotfile" at the command line.
 
 # The location of the build configuration file.
 buildconfig = "//build/config/BUILDCONFIG.gn"
 
 # The secondary source root is a parallel directory tree where
 # GN build files are placed when they can not be placed directly
 # in the source tree, e.g. for third party source trees.
@@ skipping 51 matching lines @@
   "//ios/*",
   "//ios_internal/*",
   "//ipc/*",
 
   #"//jingle/*",
   "//mash/*",
 
   #"//media/*",  # Lots of errors.
   #"//media/base/*",
   #"//media/capture/*",
-  #"//media/cast/*",
+  "//media/cast/*",
   "//media:media",
   "//media:shared_memory_support",
   "//media:media_unittests",
   "//media:audio_unittests",
   "//media:media_perftests",
   "//media/audio/*",
   "//media/blink/*",
   "//media/cdm/*",
   "//media/ffmpeg/*",
   "//media/gpu/*",
@@ skipping 176 matching lines @@
   "//third_party/skia/gn/shared_sources.gni",
 
   # CLD2 should be removed soon, delete this when we do.
   "//third_party/cld_2/BUILD.gn",
   "//tools/grit/grit_rule.gni",
 
   # Not gypi-to-gn.
   "//google_apis/BUILD.gn",
   "//printing/BUILD.gn",
 ]