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

device/BUILD.gn

 # Copyright 2015 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("vr/features.gni")
 import("//build/config/features.gni")
 import("//testing/test.gni")
 
 if (is_android) {
   import("//build/config/android/rules.gni")  # For generate_jni().
 }
 
 if (is_mac) {
   import("//build/config/mac/mac_sdk.gni")
 }
@@ skipping 291 matching lines @@
     ]
     srcjar_deps = [ "//device/gamepad:java_enums_srcjar" ]
   }
 
   java_cpp_enum("bluetooth_test_javagen") {
     sources = [
       "bluetooth/test/bluetooth_test.h",
     ]
   }
 }