Add BUILD.gn files for chromecast/media

build/config/features.gni

Index: build/config/features.gni
diff --git a/build/config/features.gni b/build/config/features.gni
index 7ed70342617c9c026accb82efe73f95c008f2438..dd7b0819b55e4f5258e5e6789549cbf91a645c0b 100644
--- a/build/config/features.gni
+++ b/build/config/features.gni
@@ -80,6 +80,10 @@ declare_args() {
 
   # Enable hole punching for the protected video.
   enable_video_hole = is_android
+
+  # Enables browser side Content Decryption Modules. Required for embedders
+  # (e.g. Android and ChromeCast) that use a browser side CDM.
+  enable_browser_cdms = is_android
 }
 
 # Additional dependent variables -----------------------------------------------
@@ -102,8 +106,6 @@ enable_spellcheck = !is_android
 
 enable_pepper_cdms = enable_plugins && (is_linux || is_mac || is_win)
 
-enable_browser_cdms = is_android
-
 # Enable basic printing support and UI.
 enable_basic_printing = !is_chromeos