Move enable extensions define to a build flag.

chrome/common/extensions/BUILD.gn

 # Copyright 2016 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("//tools/json_schema_compiler/json_features.gni")
 
 assert(enable_extensions)
 
 json_features("extension_features_unittest") {
   feature_class = "APIFeature"
   provider_class = "UnittestFeatureProvider"
   sources = [
     "//chrome/test/data/extensions/extension_api_unittest/api_features.json",
   ]
 }
 
 group("test_features") {
   public_deps = [
     ":extension_features_unittest",
   ]
 }