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This is needed so that the network conditions can be visibile to
GenericURLRequestJob in headless/public/util. We need to store
it in the HeadlessBrowserContextImpl, since GenericURLRequestJob
does not reuse the network stack (e.g. DevToolsNetworkTransaction,
etc).

Bug: 
Change-Id: I531acda73bffad2e504efbfbaa4fd8df2ef4b23e
Reviewed-on: https://chromium-review.googlesource.com/696887
Commit-Queue: Iris Uy <irisu@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: David Vallet <dvallet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506980}