[NativeFS] Refactor safe browsing code to decouple it from DownloadItem.

This is one step towards being able to run safe browsing download
protection checks on files written by the native file system API.

Bug: 995972

Change-Id: I8b97d5c41604b24d18c8790e179cce9cc0bf5eb9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1764432
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Daniel Rubery <drubery@chromium.org>
Reviewed-by: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691280}
13 files changed