Network servicification: Plumb ResourceContext through WebRequestProxyingURLLoaderFactory.

This CL plumbs the ResourceContext to the WebRequestInfo structure through
WebRequestProxyingURLLoaderFactory. This is necessary to fix
NTPInterceptionWebRequestAPITest.NTPRendererRequestsHidden for the network
servicification path.

BUG=797461, 721414

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I2957c1ece0f8f7b44f15156aeb5e554b3a91ff01
Reviewed-on: https://chromium-review.googlesource.com/1029549
Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554556}