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This includes defining a new NetLog event (SAFE_BROWSING_PING), which begins
when a ping is queued up, and ends when the result callback is fired. Covers
both the SafeBrowsingHit and ThreatDetails pings.

BUG=648323

Review-Url: https://codereview.chromium.org/2361963002
Cr-Commit-Position: refs/heads/master@{#422849}