Move enable extensions define to a build flag.

extensions/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/config/features.gni")
 import("//build/json_schema_api.gni")
 import("//extensions/common/api/schema.gni")
+import("//extensions/features/features.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
 
 assert(enable_extensions)
 
 json_schema_api("generated_api") {
   sources = extensions_api_schema_files
   schemas = true
   bundle = true
   bundle_name = ""
   root_namespace = extensions_api_root_namespace
   uncompiled_sources = extensions_api_uncompiled_sources
   uncompiled_bundle_schema_sources =
       extensions_api_uncompiled_bundle_schema_sources
+  deps = [
+    "//base",
+    "//extensions/features",
+  ]
 }
 
 mojom("mojom") {
   sources = [
     "mime_handler.mojom",
   ]
 }
 
 group("api") {
   public_deps = [
     ":generated_api",
     ":mojom",
+    "//extensions/features",
   ]
 }