[Download Home] Properly enqueue download requests

Trying to shoehorn pending actions into the PendingDownloadActions queue
doesn't work unless you use a unique download ID.  Just store a separate
int that tracks what downloads have been requested.

BUG=645926

Review-Url: https://codereview.chromium.org/2332213002
Cr-Commit-Position: refs/heads/master@{#418117}
2 files changed