Minidumps could potentially contain sensitive information, so we
shouldn't be serving them to the world.

R=machenbach@chromium.org

Change-Id: Idb7a3c07c160e82d707a4abb857e098363da3345
Reviewed-on: https://chromium-review.googlesource.com/1015802Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52670}