Migrate AssistRanker to SimpleURLLoader

As part of the effort to bring the new Network Service online, we are
deprecating URLFetcher in favor of SimpleURLLoader.

Background: https://docs.google.com/document/d/1YZDPeg3bf46QPU_vUotFwOEPHquww36b-UdRlO3ZpMU/edit

Bug: 844937
Change-Id: I4a00c42b481f10c03657aa52b9a3d197febdc6ed
Reviewed-on: https://chromium-review.googlesource.com/1070110
Reviewed-by: Andrew Moylan <amoylan@chromium.org>
Reviewed-by: Rachel Blum <groby@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Mark Pilgrim <pilgrim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561845}
17 files changed