Remove the LOG_ERROR_REPORT log severity level.

It's been unused since r255987 (which removed the --enable-dcheck flag,
among other things).

Also remove the --silent-dump-on-dcheck flag (because now there's not
even code that pretends to use it).

R=brettw@chromium.org,sky@chromium.org
BUG=352378

Review URL: https://codereview.chromium.org/331143007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@277736 0039d316-1c4b-4281-b951-d872f2087c98