[Chromecast] Allow |enable_pepper_cdms| to be set in gn args.

build/config/features.gni

Index: build/config/features.gni
diff --git a/build/config/features.gni b/build/config/features.gni
index 50993dfc7d1dd345641269b97f15162fd9d1a2b8..16b5b3be9de14cf225141bb3499836f5ddf26010 100644
--- a/build/config/features.gni
+++ b/build/config/features.gni
@@ -93,6 +93,10 @@ declare_args() {
   # (e.g. Android and ChromeCast) that use a browser side CDM.
   enable_browser_cdms = is_android
 
+  # Enables the use of CDMs in pepper plugins. Chromecast requires that this
+  # can be explicitly disabled in gn args.
+  enable_pepper_cdms = enable_plugins && (is_linux || is_mac || is_win)
+
   # Variable safe_browsing is used to control the build time configuration
   # for safe browsing feature. Safe browsing can be compiled in 4 different
   # levels: 0 disables it, 1 enables it fully, and 2 enables only UI and
@@ -129,8 +133,6 @@ enable_spellcheck = !is_ios
 # Use the operating system's spellchecker rather than hunspell.
 use_platform_spellchecker = is_android || is_mac
 
-enable_pepper_cdms = enable_plugins && (is_linux || is_mac || is_win)
-
 # Enable basic printing support and UI.
 enable_basic_printing = !is_chromeos