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On iOS 12 URLSession:dataTask:didReceiveData: delegate method can be
called when task is in NSURLSessionTaskStateCompleted state. This
resulted in extra URLFetcherResponseWriter::Finish call (one from
didReceiveData: and one from didCompleteWithError:).
URLFetcherResponseWriter::Finish DCHECKs if Finish is called twice, so
the test was flaky.

This CL changes DownloadTaskImpl to only call
URLFetcherResponseWriter::Finish from didCompleteWithError:.

Originally Reviewed-on: https://chromium-review.googlesource.com/c/1354233

Bug: 873204
Change-Id: Idece867d9d27c46a0deb57096144ed62df056901
Reviewed-on: https://chromium-review.googlesource.com/c/1355535Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613165}