Clean up download code:

Clean up download code:

- move filename generation parts and related code out of DownloadManager
  download_util is not the best place for it, but it's better than
  DownloadManager which we're trying to minimize

- clean up posting tasks to different threads. There's no need to create
  separate methods for that, it was cluttering the file needlessly

- other minor cleanups, like using static_cast instead of C-style cast

TEST=unit_tests, browser_tests, ui_tests
BUG=48913
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=55088

[ahendrickson, 2010-08-03 17:20:51]

LGTM.

[Paweł Hajdan Jr., 2010-08-03 21:07:17]

Darin: ping.