Add a stat for dragging a download from the shelf.

Bug: 783340
Change-Id: I4b34e9d98627f49412bba3251820fa38445c5e63
Reviewed-on: https://chromium-review.googlesource.com/762667


Reviewed-by: Trent Apted <tapted@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Sidney San Martín <sdy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518306}