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Currently we copy hosts + other state for every outgoing request to the
UI thread before doing per-WebContents aggregation there. This can
involve a lot of copying of data around. This CL adds some initial
per-frame aggregation beforehand to reduce this.

Also, we add a browsertest for this functionality, since none previously
existed.

Bug: 813445
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I33438bdd5d4cab9caf3fcd07419fef3f7153935e
Reviewed-on: https://chromium-review.googlesource.com/951743Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542157}