Generate notification for cached v2 download

Cached v2 image downloads were not generating notifications.
For now use a 'size_checked_iter' to generate a
notification.

Fix for bug 1132755.

Change-Id: Iec01efb93d7a1853973d355221d06fad9631afd7
Merge "Generate notification for cached v2 download"
Merge remote-tracking branch 'm_src_repo/webob-issue' into build-temp