Implement ReportingObserver.takeRecords().

This method allows queued reports to be captured early.

https://wicg.github.io/reporting/#dom-reportingobserver-takerecords

Bug: 731810
Change-Id: I885d53a1800959d39e9451fc014ce05a4cda475f
Reviewed-on: https://chromium-review.googlesource.com/1106642
Commit-Queue: Paul Meyer <paulmeyer@chromium.org>
Reviewed-by: Nate Chapin <japhet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569155}
5 files changed