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The test was failing because a failed DCHECK.
When should_terminate is true, we should never set the potential
download file length as should_terminate means the SourceStream has
found its data block has been overwritten by others. As a result,
there is no way that this source stream is reaching the end of the
file.

BUG=810982

Change-Id: I91534436a7bebfc195d81d049f917bb685cc5281
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1817153
Commit-Queue: Min Qin <qinmin@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699554}