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Similar to regular downloads, we want SB pings for requestors whose
URLs match the SB whitelist in flash downloads. To achieve that, we
sample 1% of whitelisted PPAPI downloads based on the
SafeBrowsingService::whitelist_sample_rate() if the user is in
extended reporting group and not in incognito mode.

Also refactored download_protection_service_unittest to elliminate
the use of MessageLoop (deprecated).

BUG=610924

Review-Url: https://codereview.chromium.org/2146703002
Cr-Commit-Position: refs/heads/master@{#406101}