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

device/vr/BUILD.gn

Index: device/vr/BUILD.gn
diff --git a/device/vr/BUILD.gn b/device/vr/BUILD.gn
index 27cd5c38765bd3564724f6d43cc5aa7f619d1851..54fb4a3affb86b8f6b49a2d6ea44812de7ff60cf 100644
--- a/device/vr/BUILD.gn
+++ b/device/vr/BUILD.gn
@@ -2,13 +2,20 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import("//build/config/features.gni")
+import("features.gni")
+import("//build/buildflag_header.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
 
 if (is_android) {
   import("//build/config/android/rules.gni")  # For generate_jni().
 }
 
+# Generate a buildflag header for compile-time checking of WebVR support.
+buildflag_header("features") {
+  header = "features.h"
+  flags = [ "ENABLE_WEBVR=$enable_webvr" ]
+}
+
 component("vr") {
   output_name = "device_vr"