Migrate profile_browsertest.cc to SimpleURLLoader

It was using URLFetcher. In order to make it eventually work with the
future network service we need to transition it to SimpleURLLoader
API.

The tests was extensively using the URLFetcher* naming so I decided to
replace them all by references to SimpleURLLoader instead.

Bug: 844918
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: I9f0412bcc4b5731a50e691602fa93d5ef06fba8e
Reviewed-on: https://chromium-review.googlesource.com/1177385Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Commit-Queue: Sergio Villar <svillar@igalia.com>
Cr-Commit-Position: refs/heads/master@{#584824}

testing/buildbot/filters/mojo.fyi.network_browser_tests.filter

@@ -16,10 +16,6 @@
 # These tests use URLFetchers directly.
 # https://crbug.com/853798
 -ProfileBrowserTest.SeparateMediaCache
--ProfileBrowserTest.URLFetcherUsingMainContextDuringIncognitoTeardown
--ProfileBrowserTest.URLFetcherUsingMainContextDuringShutdown
--ProfileBrowserTest.URLFetcherUsingMediaContextDuringShutdown
--ProfileWithoutMediaCacheBrowserTest.NoSeparateMediaCache
 
 # about:net-internals should be largely removed before shipping the network
 # service.