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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 WebRtcEventLogUploaderImpl et al away from URLFetcher.

Note that this CL slightly changes the flow of the WebRtcEventLogUploader code.
Previously, an URLRequestContextGetter instance was acquired on UI thread,
passed to the IO-capable task runner, used there, and ultimately deleted either
on the UI thread or on the IO-capable task runner thread, depending on the
termination circumstance (eg BrowserProcessImpl shutdown or upload completion,
respectively).

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.
Given this scenario, a natural approach to tackle this migration would be
the use of CrossThreadSharedURLLoaderFactory{Info} classes. However, as
mentioned earlier, the fact that WebRtcEventLogUploader instances are
constructed the aforementioned IO-capable task runner thread and the
deletion thread varies, make the use of CrossThreadSharedURLLoaderFactory{Info}
less straightforward.

This CL makes use of the thread agnostic network::mojom::URLLoaderFactoryPtrInfo
class instead: from the IO-capable task runner it posts a message to the UI thread
to bind a network::mojom::URLLoaderFactory object. As per the mojo documentation [1]:

  "Once the LoggerPtr is bound we can immediately begin calling Logger interface
  methods on it, which will immediately write messages into the pipe. These
  messages will stay queued on the receiving end of the pipe until someone
  binds to it and starts reading them."

[1] https://chromium.googlesource.com/chromium/src/+/master/mojo/public/cpp/bindings/README.md#Binding-an-Interface-Request

BUG=773295,873187

Change-Id: Ic29222ea63e90dcb26f3124bdea1c627570dc04d
Reviewed-on: https://chromium-review.googlesource.com/1171622
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Elad Alon <eladalon@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584048}