Pass file_length to UpdateArchiveAnalyzer

Since it's not possible to truncate a file from within a sandbox, we
need to track the file size independently, and only consider the
first |file_length| bytes of the file.

TESTED=Manually, by printing the hashes on chrome://safe-browsing, and
verifying they are correct. It's currently impossible to test this
automatically, since our unit tests do not run in the sandbox.

Bug: 944633
Change-Id: I99c84a69e1ce4d4a08e5bd92d6a1b63e807b0233
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1535258


Reviewed-by: Satoru Takabayashi <satorux@chromium.org>
Reviewed-by: Varun Khaneja <vakh@chromium.org>
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#645810}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 662e1489291632b12980347ba5d9bc77844f6e68