The right thing to do would be to ensure that the multiplication can't
overflow, but that'd be a major change bordering on a rewrite, and
would take too much time. So instead, we just instruct UBSan to look
the other way.
BUG=chromium:634834
Review-Url: https://codereview.webrtc.org/2318083002
Cr-Commit-Position: refs/heads/master@{#14154}