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This CL deduplicates code that used to be repeated in
FileURLLoaderFactory and ContentURLLoaderFactory: the overridden Clone,
private OnDisconnect method and |receivers_| field.

In the future, other aspects of URLLoaderFactory implementation (e.g.
handling of CORS and/or helpers for responding with file contents) may
hopefully also be moved into NonNetworkURLLoaderFactoryBase.

Bug: 1106995, 1105256
Change-Id: I7b8783bc4020d914fb7bdd9f56e2e9698fd9a615
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2357559
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805549}