Note for repro steps:
Queuing another page is not needed. Just press "DOWNLOAD PAGE LATER" for the page that doesn't exist.

It seems that we simply do nothing in PrerenderContents::DidFinishNavigation when navigation_handle->IsErrorPage().

This might be related to patch https://codereview.chromium.org/2653023002. The author made the following comment regarding error page:
  IsErrorPage() is added to maintain the same behavior of DidFinishNavigation as
DidNavigateMainFrame (since the latter doesn't fire it for errors).

  For PrerenderContents, in case of an error, we actually don't get an
IsErrorPage() to true unless in some races. The timing is that there are two
DidStartNavigation/DidFinishNavigation sets of callbacks. The first one is for
the load that doesn't find the page. It's not the error page yet so
IsErrorPage() is false. The second pair is for the loading of the error page, so
IsErrorPage() is true. The issue is that the renderer sends the prefetch
complete message that you added recently between these two pairs, so usually
with the thread hopping/loading times the second pair is not seen.

Based on my debugging, DidStartNavigation/DidFinishNavigation is only triggered once from PrerenderContents. In DidFinishNavigation, IsErrorPage() is true.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2d54317a97881484713af4069f01677e924dc87e

commit 2d54317a97881484713af4069f01677e924dc87e
Author: chili <chili@chromium.org>
Date: Wed Apr 12 18:10:55 2017

[Offline pages] Add new Error page failure status inside the MHTML archiver and update corresponding histogram.

This should record number of error pages we are seeing being recorded by Prerender and also eliminate this altogether.

BUG= 709639 ,692688

Review-Url: https://codereview.chromium.org/2810913002
Cr-Commit-Position: refs/heads/master@{#464087}

[modify] https://crrev.com/2d54317a97881484713af4069f01677e924dc87e/chrome/browser/android/offline_pages/offline_page_mhtml_archiver.cc
[modify] https://crrev.com/2d54317a97881484713af4069f01677e924dc87e/chrome/browser/android/offline_pages/offline_page_mhtml_archiver.h
[modify] https://crrev.com/2d54317a97881484713af4069f01677e924dc87e/chrome/browser/android/offline_pages/offline_page_mhtml_archiver_unittest.cc
[modify] https://crrev.com/2d54317a97881484713af4069f01677e924dc87e/components/offline_pages/core/offline_page_archiver.h
[modify] https://crrev.com/2d54317a97881484713af4069f01677e924dc87e/components/offline_pages/core/offline_page_model_impl.cc
[modify] https://crrev.com/2d54317a97881484713af4069f01677e924dc87e/components/offline_pages/core/offline_page_types.h
[modify] https://crrev.com/2d54317a97881484713af4069f01677e924dc87e/tools/metrics/histograms/histograms.xml

Chili fixed this, assigning it to her so she gets credit for fixing it, and marking it fixed.