Add argument to write mb error to json output.

Change-Id: I71c5fb1516c2b5bf6c6c2180c51a18b4c9ad0f08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1696445
Commit-Queue: Debrian Figueroa <debrian@google.com>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678087}