Move enable_app_list to a buildflag.

chrome/app/theme/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("//chrome/common/features.gni")
 import("//tools/grit/grit_rule.gni")
 
 grit("theme_resources") {
   source = "theme_resources.grd"
+  defines = chrome_grit_defines
   outputs = [
     "grit/theme_resources.h",
     "grit/theme_resources_map.cc",
     "grit/theme_resources_map.h",
     "theme_resources_100_percent.pak",
     "theme_resources_200_percent.pak",
     "theme_resources_300_percent.pak",
   ]
 
   deps = [
     ":chrome_unscaled_resources",
     "//ui/resources",
   ]
 
   output_dir = "$root_gen_dir/chrome"
 }
 
 grit("chrome_unscaled_resources") {
   source = "chrome_unscaled_resources.grd"
+  defines = chrome_grit_defines
   use_qualified_include = true
   outputs = [
     "grit/chrome_unscaled_resources.h",
     "chrome_unscaled_resources.pak",
   ]
 
   output_dir = "$root_gen_dir/chrome"
 }