Fail when saving page as MHTML provides information about the cause.

content/common/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/features.gni")
 import("//build/config/ui.gni")
 import("//media/media_options.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
 import("//ppapi/features/features.gni")
 import("//tools/ipc_fuzzer/ipc_fuzzer.gni")
@@ skipping 79 matching lines @@
     "cursors/webcursor_aurawin.cc",
     "cursors/webcursor_aurax11.cc",
     "cursors/webcursor_mac.mm",
     "cursors/webcursor_ozone.cc",
     "database_messages.h",
     "date_time_suggestion.h",
     "devtools_messages.h",
     "dom_storage/dom_storage_map.cc",
     "dom_storage/dom_storage_map.h",
     "dom_storage/dom_storage_messages.h",
+    "download/mhtml_save_status.cc",
+    "download/mhtml_save_status.h",
     "drag_event_source_info.h",
     "drag_messages.h",
     "drag_traits.h",
     "dwrite_font_proxy_messages.h",
     "edit_command.h",
     "establish_channel_params.cc",
     "establish_channel_params.h",
     "file_utilities_messages.h",
     "fileapi/file_system_messages.h",
     "font_cache_dispatcher_win.cc",
@@ skipping 497 matching lines @@
     "//ui/gfx/geometry/mojo",
     "//ui/gfx/mojo",
     "//url/mojo:url_mojom_gurl",
     "//url/mojo:url_mojom_origin",
   ]
 
   export_class_attribute = "CONTENT_EXPORT"
   export_define = "CONTENT_IMPLEMENTATION=1"
   export_header = "content/common/content_export.h"
 }