Remove references to extensions from //components/guest_view/.

components/guest_view/browser/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("//extensions/features/features.gni")
-
-assert(enable_extensions)
+# Currently, GuestViews are only used by extensions, so we have this
+# assert to prevent the accidental building of GuestViews on mobile
+# platforms. If you're now using GuestViews on mobile, go ahead and
+# remove this assert.
+assert(!is_android && !is_ios)
 
 static_library("browser") {
   output_name = "guest_view_browser"
   sources = [
     "//components/guest_view/browser/guest_view.h",
     "//components/guest_view/browser/guest_view_base.cc",
     "//components/guest_view/browser/guest_view_base.h",
     "//components/guest_view/browser/guest_view_event.cc",
     "//components/guest_view/browser/guest_view_event.h",
     "//components/guest_view/browser/guest_view_manager.cc",
@@ skipping 34 matching lines @@
 source_set("unit_tests") {
   testonly = true
   sources = [
     "guest_view_manager_unittest.cc",
   ]
   deps = [
     ":test_support",
     "//testing/gtest",
   ]
 }