Move enable_app_list to a buildflag.

chrome/test/data/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("//chrome/common/features.gni")
 import("//tools/grit/grit_rule.gni")
 
 grit("webui_test_resources") {
   source = "webui_test_resources.grd"
+  defines = chrome_grit_defines
   use_qualified_include = true
   outputs = [
     "test/data/grit/webui_test_resources.h",
     "webui_test_resources.pak",
   ]
   output_dir = "$root_gen_dir/chrome"
 }