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This lets a C++ embedder tell if a request was sent from the browser
or the renderer.

Bug: 
Change-Id: Id191dbc3003facc5125b17f38922b4d59ed67811
Reviewed-on: https://chromium-review.googlesource.com/746646Reviewed-by: Eric Seckler <eseckler@chromium.org>
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512833}