Move enable extensions define to a build flag.

extensions/shell/common/api/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/json_schema_api.gni")
 import("//tools/json_schema_compiler/json_features.gni")
 
 schema_sources = [ "identity.idl" ]
 root_namespace = "extensions::shell::api::%(namespace)s"
 
 json_schema_api("api") {
   sources = schema_sources
   schemas = true
   bundle = true
   bundle_name = "Shell"
 }
 
 json_schema_api("api_registration") {
   sources = schema_sources
   impl_dir = "//extensions/shell/browser/api"
   bundle_registration = true
   bundle_name = "Shell"
 
   deps = [
     ":api",
+    "//extensions/common",
   ]
 }
 
 json_features("shell_api_features") {
   feature_class = "APIFeature"
   provider_class = "ShellAPIFeatureProvider"
   sources = [
     "../../../common/api/_api_features.json",
     "_api_features.json",
   ]
@@ skipping 24 matching lines @@
 }
 
 group("extensions_features") {
   public_deps = [
     ":shell_api_features",
     ":shell_behavior_features",
     ":shell_manifest_features",
     ":shell_permission_features",
   ]
 }