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This patch is preparation for allowing signin to work with the Network
Service as in that path there will be no direct access to the
net::URLRequest object. These wrapper classes expose the minimum
interface needed by the signin module to modify headers.

Bug: 789670
Change-Id: I06bc2d99dba2de8359ab2f93d9a220f6c6e3b502
Reviewed-on: https://chromium-review.googlesource.com/1098377
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569339}