crash-reporter: Add session_manager logs to Chrome crashes.

Attach the last twenty session_manager messages from
/var/log/messages to Chrome crash reports.

BUG=chromium:233833
TEST=manual: triggered a crash and checked chrome.txt in the
     crash report (1f8c6ff259947e7a)

Change-Id: I733b448ccc6e01111556a5a632e185cc58dfd917
Reviewed-on: https://chromium-review.googlesource.com/243378


Tested-by: Dan Erat <derat@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Chris Masone <cmasone@chromium.org>
Commit-Queue: Dan Erat <derat@chromium.org>