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

chrome/browser/android/vr_shell/BUILD.gn

 # Copyright 2016 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/android/rules.gni")
 import("//chrome/common/features.gni")
+import("//device/vr/features.gni")
 import("//testing/test.gni")
 
 assert(enable_webvr)
 
 if (current_cpu == "arm" || current_cpu == "arm64") {
   static_library("vr_common") {
     defines = []
 
     sources = [
       "android_ui_gesture_target.cc",
@@ skipping 92 matching lines @@
       ":vr_common",
       "//base/test:run_all_unittests",
       "//base/test:test_support",
       "//chrome/browser",
       "//testing/gtest",
       "//third_party/WebKit/public:blink",
       "//ui/gfx/geometry",
     ]
   }
 }