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

content/common/download/mhtml_save_status.h

Index: content/common/download/mhtml_save_status.h
diff --git a/content/common/download/mhtml_save_status.h b/content/common/download/mhtml_save_status.h
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+++ b/content/common/download/mhtml_save_status.h
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+// 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.
+
+#ifndef CONTENT_COMMON_DOWNLOAD_MHTML_SAVE_STATUS_H_
+#define CONTENT_COMMON_DOWNLOAD_MHTML_SAVE_STATUS_H_
+
+namespace content {
+
+// Status result enum for MHTML generation.
+enum class MhtmlSaveStatus {
+  SUCCESS = 0,
+
+  // Could not properly close the file where data was written to. Determined by
+  // the browser.
+  FILE_CLOSING_ERROR,
+
+  // Could not create the file that would be written to. Determined by the
+  // browser.
+  FILE_CREATION_ERROR,
+
+  // Could not write serialized data to the file. Determined by the renderer.
+  FILE_WRITTING_ERROR,
+
+  // The DOM changed and a previously existing frame is no more. Determined by
+  // the browser.
+  FRAME_NO_LONGER_EXISTS,
+
+  // The main frame was not allowed to be serialized by either policy or cache
+  // control. Determined by the renderer.
+  FRAME_SERIALIZATION_FORBIDDEN,
+
+  // A render process needed for the serialization of one of the page's frame is
+  // no more. Determined by the browser.
+  RENDER_PROCESS_EXITED,
+
+  // NOTE: always keep this entry at the end and add new status types only
+  // immediately above this line. Set LAST to the new last value and update the
+  // implementation of GetMhtmlSaveStatusLabel. Make sure to update the
+  // corresponding histogram enum accordingly.
+  LAST = RENDER_PROCESS_EXITED,
+};
+
+// Gets a textual representation of the provided MhtmlSaveStatus value.
+const char* GetMhtmlSaveStatusLabel(MhtmlSaveStatus save_status);
+
+}  // namespace content
+
+#endif  // CONTENT_COMMON_DOWNLOAD_MHTML_SAVE_STATUS_H_