Get rid of RequestRegistry (part 2): remove cancellation methods.

Get rid of RequestRegistry (part 2): remove cancellation methods.

* CancelForFilePath is used only from drive::JobScheduler, and is now 100%
  covered by the new, base::Callback based cancellation.
* I guess CancelAll had been used just for satisfing the  DCHECK in ~RequestRegistry.
  But it's not needed. The requests are discarded without problem in the destructor.

BUG=164098
R=satorux@chromium.org, tzik@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=207185

[kinaba, 2013-06-19 03:14:46]

+tzik: c/b/sync_file_system
+satorux: c/b/contacts and everything else

[tzik, 2013-06-19 03:35:05]

lgtm

[satorux1, 2013-06-19 03:39:22]

LGTM

[commit-bot: I haz the power, 2013-06-19 03:45:04]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/kinaba@chromium.org/17385023/7001

[kinaba, 2013-06-19 05:23:41]

Committed patchset #2 manually as r207185 (presubmit successful).