[Downloads] Avoid resetting SecureHash state across an interruption.

DownloadFileImpl/BaseFile was causing the SecureHash object to be
finalized when a download was interrupted. This caused the hash state to
be incorrect and subsequently the hash resulting from resuming the
download was also incorrect.

BUG=7648
BUG=581164

Review URL: https://codereview.chromium.org/1591523002

Cr-Commit-Position: refs/heads/master@{#371428}
4 files changed