Added UMA for tracking the cancelled downloads

Bug: 1052544
Change-Id: I173a4e3c364fc6f69e4dd24b934f7ed50a50572e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2058063


Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Shakti Sahu <shaktisahu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743328}