[Merge to 49] [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}
(cherry picked from commit bdc7250e539a9f40f84fd157bc29481e5aac2bf8)

Review URL: https://codereview.chromium.org/1648683002 .

Cr-Commit-Position: refs/branch-heads/2623@{#177}
Cr-Branched-From: 92d77538a86529ca35f9220bd3cd512cbea1f086-refs/heads/master@{#369907}
4 files changed