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

build/config/BUILD.gn

 # Copyright (c) 2013 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/allocator.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/chromecast_build.gni")
 import("//build/config/crypto.gni")
 import("//build/config/dcheck_always_on.gni")
 import("//build/config/features.gni")
@@ skipping 122 matching lines @@
     defines += [ "OFFICIAL_BUILD" ]
   }
   if (is_chrome_branded) {
     defines += [ "GOOGLE_CHROME_BUILD" ]
   } else {
     defines += [ "CHROMIUM_BUILD" ]
   }
   if (enable_media_router) {
     defines += [ "ENABLE_MEDIA_ROUTER=1" ]
   }
-  if (enable_webvr) {
-    defines += [ "ENABLE_WEBVR" ]
-  }
   if (is_syzyasan) {
     defines += [
       "SYZYASAN",
       "MEMORY_SANITIZER_INITIAL_SIZE",
     ]
   }
   if (!fieldtrial_testing_like_official_build && !is_chrome_branded) {
     defines += [ "FIELDTRIAL_TESTING_ENABLED" ]
   }
 
@@ skipping 219 matching lines @@
       # then delete the precompile.c.obj file, then build again.
       #
       # TODO(sof): determine VS2015 status and retire the setting from all
       # precompiled configurations.
       cflags_c = [ "/wd4206" ]
     } else if (is_mac) {
       precompiled_source = "//build/precompile.h"
     }
   }
 }