Convert WebKitBrowserTest to use URLLoaderInterceptor

This test was using a net::URLRequestInterceptor so was not actually
working correctly with network service. The interceptor it was using
also assumes it is on the IO thread which was causing issues when
running NS in-process on its own thread.

Bug: 908993
Change-Id: I009ddf13b53cecac9615c4d2019405ddede57c69
Reviewed-on: https://chromium-review.googlesource.com/c/1357405
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612879}
4 files changed