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URLFetcher will stop working with advent of Network Service, and
SimpleURLLoader is the replacement API for most clients.
This CL migrates HttpBridge and FinancialPing away from URLFetcher.

Note that this CL slightly changes the flow of the HttpBridge{Factory} code.
Previously, an URLRequestContextGetter instance was acquired on UI thread,
passed to the Sync and IO threads.
However, URLLoaderFactory and SimpleURLLoader have stricter threading
restrictions than URLFetcher and URLRequestContextGetter. For instance, a
SharedURLLoaderFactory instance needs to be created, used and deleted on
the same thread.

For this reason methods like HttpBridgeFactory::OnSignalReceived
can not simply null out |url_loader_factory_| which is created on the "sync"
thread.

Additionally, some preexisting tests do not seem relevant for the context
of network service, and got removed, eg TestUsesSameHttpNetworkSession,
RequestContextGetterReleaseOrder and EarlyAbortFactory.
Particularly, the last two used to deal with URLRequestContextGetter
destruction. With the URLLoader migration, HttpBridge now holds on
to mojo pipes  instead of URLRequestContextGetter.

BUG=773295,844968

Change-Id: I185f814ba171dca69fec55b913b86a0cc9993ef1
Reviewed-on: https://chromium-review.googlesource.com/1174655
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Nicolas Zea <zea@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586115}