Add BUILD.gn files for chromecast/media

chromecast/chromecast.gni

Index: chromecast/chromecast.gni
diff --git a/third_party/widevine/cdm/widevine.gni b/chromecast/chromecast.gni
similarity index 57%
copy from third_party/widevine/cdm/widevine.gni
copy to chromecast/chromecast.gni
index b10a92b47494fe4954df89e40b9e4e80a12fb653..b6f7307bcce064e4d296e9163cf4b31dc1db7ca9 100644
--- a/third_party/widevine/cdm/widevine.gni
+++ b/chromecast/chromecast.gni
@@ -3,6 +3,8 @@
 # found in the LICENSE file.
 
 declare_args() {
-  # Allow widevinecdmadapter to be built in Chromium.
-  enable_widevine = false
+  # is_chromecast_chrome_branded is set to true for an official build
+  is_chromecast_chrome_branded = false
+
+  use_playready = false
 }