Add slice information to DownloadDestinationObserver::DestinationUpdate().

When destination updates, the slice information should also be sent to the observer.
So that the observer can inform the history db about the changes.

BUG=644352

Review-Url: https://codereview.chromium.org/2722503006
Cr-Commit-Position: refs/heads/master@{#454140}
7 files changed