Move enable extensions define to a build flag.

build/config/BUILD.gn

 # Copyright (c) 2013 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/allocator.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/chromecast_build.gni")
 import("//build/config/crypto.gni")
 import("//build/config/dcheck_always_on.gni")
 import("//build/config/features.gni")
@@ skipping 143 matching lines @@
   }
   if (use_external_popup_menu) {
     defines += [ "USE_EXTERNAL_POPUP_MENU=1" ]
   }
   if (enable_webrtc) {
     defines += [ "ENABLE_WEBRTC=1" ]
   }
   if (!enable_nacl) {
     defines += [ "DISABLE_NACL" ]
   }
-  if (enable_extensions) {
-    defines += [ "ENABLE_EXTENSIONS=1" ]
-  }
   if (enable_task_manager) {
     defines += [ "ENABLE_TASK_MANAGER=1" ]
   }
   if (enable_themes) {
     defines += [ "ENABLE_THEMES=1" ]
   }
   if (enable_rlz) {
     defines += [ "ENABLE_RLZ" ]
   }
   if (enable_image_loader_extension) {
@@ skipping 259 matching lines @@
       # file doesn't exist, no error will be generated (probably MS tested this
       # case but forgot the other one?). To reproduce this error, do a build,
       # then delete the precompile.c.obj file, then build again.
       cflags_c = [ "/wd4206" ]
     } else if (is_mac) {
       precompiled_header = "build/precompile.h"
       precompiled_source = "//build/precompile.h"
     }
   }
 }