Add BUILD.gn files for chromecast/media

Set default chromecast args in chromecast/args.gn

BUG=

Review URL: https://codereview.chromium.org/1055173005

Cr-Original-Commit-Position: refs/heads/master@{#326133}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 4025129eb8db73777a99731cae1aef839933b366

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