Convert enable_plugins to a buildflag header.

ppapi/features/features.gni

Index: ppapi/features/features.gni
diff --git a/ppapi/features/features.gni b/ppapi/features/features.gni
index b28feac0fdf00acc0179731028e0d6be442b06a6..559373e9794b8f2f4e1ce7acc535f4e656af8b57 100644
--- a/ppapi/features/features.gni
+++ b/ppapi/features/features.gni
@@ -2,8 +2,13 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/config/chromecast_build.gni")
 import("//build/config/features.gni")
 
+declare_args() {
+  enable_plugins = (!is_android && !is_ios) || is_chromecast
+}
+
 # Enables the use of CDMs in pepper plugins.
 #
 # This is available as a buildflag for C++ preprocessor, see