crash-reporter: Attach Chrome logs to crash reports.

When Chrome crashes, gather the last 20 lines of each of the
latest two log files in /var/log/chrome and
/home/chronos/user/log and include them in the archive file
that's attached to crash reports.

BUG=chromium:405732
TEST=triggered crashes at the login screen and after logging
     in and verified that logs were included in crash
     reports: bdf766f0d6d1e066, a9d410e1a86f996b

Change-Id: I09e3cee23af108dc216d64aae85d78751d5649d4
Reviewed-on: https://chromium-review.googlesource.com/216427


Reviewed-by: Ben Chan <benchan@chromium.org>
Tested-by: Daniel Erat <derat@chromium.org>
Commit-Queue: Daniel Erat <derat@chromium.org>