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This CL introduces CastExtensionURLLoaderFactory::Create static method
that allows creating an CastExtensionURLLoaderFactory 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<CastExtensionURLLoaderFactory>, 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 CastExtensionURLLoaderFactory 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: I6802fe556698bcba84e8332bcaae4665410c2f82
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2410579
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Sean Topping <seantopping@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810927}