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

gpu/vulkan/BUILD.gn

 # Copyright 2016 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("features.gni")
+import("//build/buildflag_header.gni")
 import("//testing/test.gni")
 
-config("vulkan_config") {
-  defines = [ "ENABLE_VULKAN" ]
+# Generate a buildflag header for compile-time checking of Vulkan support.
+buildflag_header("features") {
+  header = "features.h"
+  flags = [ "ENABLE_VULKAN=$enable_vulkan" ]
 }
-
 if (enable_vulkan) {
   vulkan_lib_dir = getenv("VULKAN_SDK") + "/lib"
   component("vulkan") {
     output_name = "vulkan_wrapper"
 
     if (is_linux) {
       assert(use_x11, "Vulkan only support x11 at this point.")
       sources = [
         "vulkan_command_buffer.cc",
         "vulkan_command_buffer.h",
@@ skipping 21 matching lines @@
         "vulkan_shader_module.h",
         "vulkan_surface.cc",
         "vulkan_surface.h",
         "vulkan_swap_chain.cc",
         "vulkan_swap_chain.h",
       ]
 
       configs += [ "//build/config:precompiled_headers" ]
       defines = [ "VULKAN_IMPLEMENTATION" ]
 
-      all_dependent_configs = [
-        ":vulkan_config",
-        "//third_party/vulkan:vulkan_headers",
-      ]
+      all_dependent_configs = [ "//third_party/vulkan:vulkan_headers" ]
       libs = [ "vulkan" ]
 
       if (current_cpu == "x64") {
         lib_dirs = [
           "/usr/lib/x86_64-linux-gnu",
           vulkan_lib_dir,
         ]
       } else {
         assert(false, "Unsupported vulkan target: " + current_cpu)
       }
@@ skipping 39 matching lines @@
       "//testing/gtest",
     ]
 
     if (use_x11) {
       sources += [ "tests/native_window_x11.cc" ]
       deps += [ "//ui/gfx/x" ]
       configs += [ "//build/config/linux:x11" ]
     }
   }
 }