UMA for failed MHTML loading including offline namespace

Propagates a status code from MHTMLArchive::Create back to the browser so that
OfflinePageTabHelper can report the result of trying to load the archive along
with the page's offline namespace.

Bug: 905409
Change-Id: I75df2dc65498598a6c3ce676899b8b9dab872822
Reviewed-on: https://chromium-review.googlesource.com/c/1336216
Reviewed-by: Dmitry Gozman <[email protected]>
Reviewed-by: Scott Violet <[email protected]>
Reviewed-by: Carlos Knippschild <[email protected]>
Reviewed-by: Jian Li <[email protected]>
Reviewed-by: Daniel Cheng <[email protected]>
Reviewed-by: Brian White <[email protected]>
Commit-Queue: Ian Wells <[email protected]>
Cr-Commit-Position: refs/heads/master@{#630064}
22 files changed