Convert enable_plugins to a buildflag header.

tools/grit/grit_rule.gni

 # 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.
 
 # Instantiate grit. This will produce a script target to run grit, and a
 # static library that compiles the .cc files.
 #
 # Parameters
 #
 #   source (required)
@@ skipping 222 matching lines @@
     ]
   }
 }
 
 if (enable_media_router) {
   grit_defines += [
     "-D",
     "enable_media_router",
   ]
 }
-if (enable_plugins) {
-  grit_defines += [
-    "-D",
-    "enable_plugins",
-  ]
-}
 if (enable_themes) {
   grit_defines += [
     "-D",
     "enable_themes",
   ]
 }
 if (enable_webrtc) {
   grit_defines += [
     "-D",
     "enable_webrtc",
@@ skipping 249 matching lines @@
     if (defined(invoker.configs)) {
       configs += invoker.configs
     }
 
     if (defined(invoker.visibility)) {
       visibility = invoker.visibility
     }
     output_name = grit_output_name
   }
 }