Add network::SharedURLLoaderFactory setter/getter in BrowserProcess

This is a preparation CL that adds a way for tests to get a
network::SharedURLLoaderFactory instance from BrowserProcess.
Particularly, TestingBrowserProcess class can also set a
SharedURLLoaderFactory instance for testing.

It is used in [1].

[1] https://crrev.com/c/1066370



BUG=773295

Change-Id: I07fe87550abb63e0225b552f4b44ba865997534c
Reviewed-on: https://chromium-review.googlesource.com/1130616
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#573883}
5 files changed