Replace net_error_helper's content::ResourceFetcher usage with SimpleURLLoader

This patch replaces net_error_helper's content::ResourceFetcher
usage with SimpleURLLoader.

It also removes resource_fetcher.h, resource_fetcher_impl.cc/h,
and resource_fetcher_browsertest.cc since these are not used
anymore.

Bug: 966323
Change-Id: I71499a071cb392e0e25c0a9ae633f000e9543ad6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1634592
Commit-Queue: Julie Jeongeun Kim <jkim@igalia.com>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667584}