Additional comments:
Note:
If you visit m.facebook.com on a different tab now and re-download the page offline, the page gets downloaded offline successfully and shows up as a different offline page in Downloads home as well

Last Good Build: 57.0.2938.0
First Bad Build: 57.0.2942.0

This issue is seen on Android devices ex. Google Pixel (7.1/NDE63V), Spice Mi-498 (6.0.1/MOB30W), Karbonn Sparkle V (5.1.1/LMY47V), Samsung Galaxy J2 (5.1.1/LMY47X), Samsung Galaxy S3 (4.3/JSS15J), Samsung Galaxy J7 (5.1.1/LMY48B), Samsung Galaxy S4 (5.0.1/LRX22C), Nexus 7 (6.0.1/MOB30X) and Nexus 9 (7.0/NRD91N)

Bisect Range: https://chromium.googlesource.com/chromium/src/+log/57.0.2938.0..57.0.2942.0?pretty=fuller&n=10000

Please find logs and Video @ http://go/chrome-androidlogs1/6/671955

Make sure you only delete post save if namespace is "last_n".

It seems that you might be deleting the previous "download" page.

Bisect script pointed to https://chromium.googlesource.com/chromium/src/+/96de1925bdf5c051bb903581a427d7f9795c0032

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3d768b7fc9d17d2df1f6d6d2d46758944e3bf09a

commit 3d768b7fc9d17d2df1f6d6d2d46758944e3bf09a
Author: carlosk <carlosk@chromium.org>
Date: Tue Dec 13 22:30:02 2016

Fix snapshots from Downloads being deleted by last_n.

Partially reverts the change that fixed good snapshots being deleted by
failed ones in the context of last_n
(https://codereview.chromium.org/2542833003/) because it created a new bug
were pages offline-d through Downloas were being incorrectly erased
( https://crbug.com/671955 ). The reverted fix will be solved again later using a
different approach.

The reason to make this revert only partial is to keep the test
improvements made which are still useful and valid. One test that had
to be disabled because of the fix being removed. A new test was also
added to confirm the incorrect snapshot deletion issue and avoid future
occurrences.

BUG= 655697 , 671955 

Review-Url: https://codereview.chromium.org/2561163002
Cr-Commit-Position: refs/heads/master@{#438314}

[modify] https://crrev.com/3d768b7fc9d17d2df1f6d6d2d46758944e3bf09a/chrome/browser/android/offline_pages/recent_tab_helper.cc
[modify] https://crrev.com/3d768b7fc9d17d2df1f6d6d2d46758944e3bf09a/chrome/browser/android/offline_pages/recent_tab_helper.h
[modify] https://crrev.com/3d768b7fc9d17d2df1f6d6d2d46758944e3bf09a/chrome/browser/android/offline_pages/recent_tab_helper_unittest.cc

This issue is fixed on latest M56-56.0.2924.47 and M57-57.0.2969.0