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Also changed the API for the prefixed version to maintain compatibility between the two.

This is an interop issue, Firefox bubbles the event while Safari does not, but I think we should match Firefox.

See http://www.w3.org/TR/page-visibility/#sec-processing-model

BUG=501821

Review URL: https://codereview.chromium.org/1435893003

Cr-Commit-Position: refs/heads/master@{#361352}