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There are 2 issues being fixed here:
1. base::ReplaceFile is used to move the downloaded file from the temp location to the desired file location. base::ReplaceFile fails if the temp file and the destination file are on different volumes. Using base::Move instead, as it doesn't have that limitation.
2. Getting the size of the downloaded file directly from the NSURLSessionTask instead of from the file system.

Bug: 1153304
Change-Id: I6d38a34a9893e2dfb59d605f657b42d381ec9e90
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2583338
Commit-Queue: Mila Green <milagreen@chromium.org>
Reviewed-by: Sorin Jianu <sorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836347}