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This is done by switching from net::URLRequestMockHTTPJob to using
net::test_server::EmbeddedTestServer.

Bug: 776589
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I8cf56477956ac64c70ba65221a1c248f39ff7944
Reviewed-on: https://chromium-review.googlesource.com/810205
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522110}