Don't trash stdout with "dropped: overflow" messages (or other errors)
in the log reader, which then cause generated json files to fail to be
read by other tools.

Change-Id: Ie27639dbbee6fc9e8da0bc6901667c3a2835fbef
Reviewed-on: https://chromium-review.googlesource.com/456499Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43901}