Add delegate to install web_contents helpers for MimeHandlerView.

extensions/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/crypto.gni")
 import("//build/config/features.gni")
 
 # GYP version: extensions/extensions.gyp:extensions_browser
 source_set("browser") {
   sources = [
@@ skipping 288 matching lines @@
       "guest_view/guest_view_constants.cc",
       "guest_view/guest_view_constants.h",
       "guest_view/guest_view_manager_factory.h",
       "guest_view/guest_view_manager.cc",
       "guest_view/guest_view_manager.h",
       "guest_view/guest_view.h",
       "guest_view/mime_handler_view/mime_handler_view_constants.cc",
       "guest_view/mime_handler_view/mime_handler_view_constants.h",
       "guest_view/mime_handler_view/mime_handler_view_guest.h",
       "guest_view/mime_handler_view/mime_handler_view_guest.cc",
+      "guest_view/mime_handler_view/mime_handler_view_guest_delegate.h",
       "guest_view/web_view/javascript_dialog_helper.cc",
       "guest_view/web_view/javascript_dialog_helper.h",
       "guest_view/web_view/web_view_constants.cc",
       "guest_view/web_view/web_view_constants.h",
       "guest_view/web_view/web_view_guest.cc",
       "guest_view/web_view/web_view_guest.h",
       "guest_view/web_view/web_view_guest_delegate.cc",
       "guest_view/web_view/web_view_guest_delegate.h",
       "guest_view/web_view/web_view_permission_types.h",
       "guest_view/web_view/web_view_permission_helper.cc",
@@ skipping 83 matching lines @@
       ]
     }
   }
 
   if (is_win) {
     cflags = [
       "/wd4267", # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
     ]
   }
 }