Move enable extensions define to a build flag.

extensions/common/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("//extensions/features/features.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
 
 source_set("common_constants") {
   sources = [
     "constants.cc",
     "constants.h",
   ]
 
   # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
   configs += [ "//build/config/compiler:no_size_t_to_int_warning" ]
@@ skipping 339 matching lines @@
       "//extensions/strings",
       "//ipc",
       "//testing/gmock",
       "//testing/gtest",
       "//ui/base",
       "//ui/gfx",
       "//url",
     ]
   }
 }  # enable_extensions