Update error checks when saving an offline page

Changes some pre-condition checks for saving an offline page. Pages saved from
the current active tab will now succeed even if they present security issues, as
for them to be opened at all the user must have gone through the warnings and
accepted the risks. Saves in the background are also changed to only fail when
certificate errors or SafeBrowsing situations are detected.

Bug: 912428, 710248
Change-Id: I0a88f590e22597ef26a195ba7344af4ec4cfb17a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1592381
Commit-Queue: Carlos Knippschild <carlosk@chromium.org>
Reviewed-by: Emily Stark <estark@chromium.org>
Reviewed-by: Dan H <harringtond@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657955}