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

chrome/browser/BUILD.gn

 # Copyright 2014 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/chrome_build.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//build/split_static_library.gni")
 import("//chrome/common/features.gni")
 import("//components/os_crypt/features.gni")
 import("//components/spellcheck/spellcheck_build_features.gni")
+import("//device/vr/features.gni")
 import("//extensions/features/features.gni")
 import("//media/media_options.gni")
 import("//net/features.gni")
 import("//ppapi/features/features.gni")
 import("//printing/features/features.gni")
 import("//rlz/features/features.gni")
 import("//sandbox/features.gni")
 import("//services/service_manager/public/service_manifest.gni")
 import("//third_party/protobuf/proto_library.gni")
 import("//ui/base/ui_features.gni")
@@ skipping 1528 matching lines @@
     "//content/public/common:features",
     "//content/public/common:service_names",
     "//courgette:courgette_lib",
     "//crypto",
     "//crypto:platform",
     "//device/base",
     "//device/bluetooth:mojo",
     "//device/power_save_blocker",
     "//device/usb/mojo",
     "//device/usb/public/interfaces",
+    "//device/vr:features",
     "//extensions/features",
     "//gin:gin_features",
     "//google_apis",
     "//gpu/config",
     "//ipc",
     "//media",
     "//media:media_features",
     "//media/midi",
     "//mojo/common",
     "//mojo/edk/system",
@@ skipping 2984 matching lines @@
       "media/pepper_cdm_test_constants.cc",
       "media/pepper_cdm_test_constants.h",
     ]
   }
 }
 
 service_manifest("preferences_manifest") {
   name = "preferences"
   source = "prefs/preferences_manifest.json"
 }