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URLFetcher et al will stop working in the browser process with advent
of Network Service, and SimpleURLLoader is the replacement API
for most clients. CL migrates TranslateURLFetcher to the new API.

Additionally, various unittests were also updated to use the new
TestURLLoaderFactory and EmbeddedTestServer machinery.

As a bonus, the CL allows us to get of URLRequestContextGetter
references throughout the TranslateURLFetcher path, including
TranslateLanguageList, TranslateScript, etc.

Since TestURLFetcherFactory allowed us to retrieve the actual
URLFetcher object (given an known 'id'), and SimpleURLLoader does
not, some helper methods were introduced, in order to verify the
state of the objects at a certain moment of the execution flow,
eg LanguageFetchURLForTesting and HasOngoingLanguageListLoadingForTesting.

Last, in a follow up CL, I will change the nomenclature used
throughout these files, from "fetcher" to "loader".

Bug: 773295,844971
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.linux:linux_mojo;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ica404dded4fab79a0035df825f12d2b1bb06329e
Reviewed-on: https://chromium-review.googlesource.com/1085947Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#567002}