Reland of move enable_vulkan from build/config, and move it to the buildflag_header system.

content/gpu/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/ui.gni")
+import("//gpu/vulkan/features.gni")
 import("//media/media_options.gni")
 import("//ui/ozone/ozone.gni")
 
 # See //content/BUILD.gn for how this works.
 group("gpu") {
   visibility = [ "//content/*" ]  # This is an internal content API.
 
   if (is_component_build) {
     public_deps = [
       "//content",
@@ skipping 85 matching lines @@
   }
 
   if (use_ozone) {
     deps += [ "//ui/ozone" ]
   }
 
   if (enable_vulkan) {
     deps += [ "//gpu/vulkan" ]
   }
 }