Convert enable_plugins to a buildflag header.

ppapi/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/features.gni")
 import("//build/config/nacl/config.gni")
 import("//build/config/nacl/rules.gni")
+import("//ppapi/features/features.gni")
 import("//testing/test.gni")
 if (is_mac) {
   import("//build/config/mac/rules.gni")
 }
 
 assert(!is_android || enable_plugins,
        "//ppapi should not be referenced when is_android && !enable_plugins")
 
 # Test files shared between several targets below.
 #
@@ skipping 449 matching lines @@
       data_deps +=
           [ ":ppapi_nacl_tests(//build/toolchain/nacl:glibc_${target_cpu})" ]
     }
 
     if (is_linux && enable_nacl_nonsfi) {
       data_deps +=
           [ ":ppapi_nacl_tests(//build/toolchain/nacl:newlib_pnacl_nonsfi)" ]
     }
   }
 }