Fix an flaky test for parallel download

This CL fixes an issue in DownloadFileWithError that
when injecting an error on a write offset, we should check if
the offset is overwritten by a write operation, rather than just
the beginning offset.
And then we use this fix to fix the flaky MiddleSliceDelayedError
as the same offset will be overwritten twice by 2 streams.

BUG=1105429, 1106059

Change-Id: Iff9b9d796ed5d86c68b24667096e2089564c4c0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2401312


Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805605}