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There are a lot of features of DMG files that we don't support, and some
of these can be used to deliver malware. We should always send a ping
when we detect a DMG file, instead of relying on detecting the contained
binary.

Bug: 882927
Change-Id: Idf7523f575a5d9778ab5d14ea1112317118fd2ed
Reviewed-on: https://chromium-review.googlesource.com/1225050Reviewed-by: Jialiu Lin <jialiul@chromium.org>
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595497}