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Currently, with HBD, if you navigate to https://get.adobe.com/flashplayer and
click "Download", it will trigger the Run Flash prompt. We should ignore
navigations if the source URL is the Flash download page itself.

BUG=656062

Review-Url: https://codereview.chromium.org/2435933002
Cr-Commit-Position: refs/heads/master@{#427005}