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

cc/BUILD.gn

Index: cc/BUILD.gn
diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index 342f566557b0ee164ad7ff919e37672dc1d4e3b6..0fd7816405ddec4e7d704c2e4f768b12e3c6533f 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn
@@ -2,8 +2,8 @@
 # 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("//cc/cc.gni")
+import("//gpu/vulkan/features.gni")
 
 cc_component("cc") {
   sources = [
@@ -531,6 +531,7 @@ cc_component("cc") {
     "//gpu/command_buffer/client:gles2_interface",
     "//gpu/ipc:gl_in_process_context",
     "//gpu/skia_bindings:skia_bindings",
+    "//gpu/vulkan:features",
     "//media",
     "//mojo/public/cpp/bindings:struct_traits",
     "//third_party/libyuv",