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

gpu/vulkan/features.gni

Index: gpu/vulkan/features.gni
diff --git a/gpu/vulkan/features.gni b/gpu/vulkan/features.gni
new file mode 100644
index 0000000000000000000000000000000000000000..f02ac88abfff9ecb60b1e08a8839c6c5b12a6ad8
--- /dev/null
+++ b/gpu/vulkan/features.gni
@@ -0,0 +1,10 @@
+# Copyright 2017 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.
+
+# Features used by targets inside and outside of |gpu/vulkan|.
+# For details see declare_args() in build/config/BUILDCONFIG.gn.
+declare_args() {
+  # Enable experimental vulkan backend.
+  enable_vulkan = false
+}