Move enable extensions define to a build flag.

chrome/common/extensions/api/BUILD.gn

 # Copyright 2015 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/ui.gni")
 import("//build/json_schema_api.gni")
 import("//chrome/common/features.gni")
+import("//extensions/features/features.gni")
 import("//tools/json_schema_compiler/json_features.gni")
 
 assert(enable_extensions)
 
 schema_sources = [
   "accessibility_features.json",
   "accessibility_private.json",
   "activity_log_private.json",
   "autofill_private.idl",
   "automation.idl",
@@ skipping 247 matching lines @@
 }
 
 group("extensions_features") {
   public_deps = [
     ":api_features",
     ":behavior_features",
     ":manifest_features",
     ":permission_features",
   ]
 }