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This CL adds enforcement that |resource_request.request_initiator| matches
|current_context.render_frame_host->GetLastCommittedOrigin()| and
rejects the prefetch request otherwise.

To gain visibility into violations occurring in the wild, the CL also
adds calls to mojo::ReportBadMessage, so that IsValidCrossOriginPrefetch
failures will generate a DumpWithoutCrashing (and potentially terminate
the renderer process that sends the invalid IPC).

Bug: 1065076
Change-Id: I8150b253b881d74d4453a243be295e1ab8b1287e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2122621
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Dominic Farolino <dom@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756839}