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This CL introduces ExternalFileURLLoaderFactory::Create static method
that allows creating a ExternalFileURLLoaderFactory that is owned by its
|receivers_| and will self-delete when the last receiver disconnects.

This CL removes the ability to directly construct and own a
std::unique_ptr<ExternalFileURLLoaderFactory>, because this ability
means that the factory can be destructed while receivers bound via the
Clone method are still alive (see the associated bug).  In particular,
this CL stops exposing ExternalFileURLLoaderFactory constructor as a
public member, which forces construction to go via the new Create static
method.

This CL mostly just follows the pattern established earlier by
https://crrev.com/c/2337411.

Bug: 1106995
Change-Id: Iadb47d1f73c73f08a8d0cd2a9c303ddef7158bad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2443398
Commit-Queue: Sam McNally <sammc@chromium.org>
Auto-Submit: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813474}