Add INTERRUPTED case in MobileDownload.Background metrics.

The STARTED count in the MobileDownload.Background metrics is more than a double
of total count of completed + cancelled + failed. This is due to a download task
might be interrupted multiple times before it finishes. To fix it, we add a
INTERRUPTED bucket in this metrics.

We can roughly estimate the crash rate by using:
(start - interrupted - completed - cancelled - failed) / start

We can't distinguish the following cases from the crash rate though:
1) A download task has been paused by user before Chrome crashes.
2) Chrome is killed by rebooting the devices / users, rather than killed by OOM.

Bug: 864625
Change-Id: Ic1f03bc665d5411c73ab16138cda259f0cab6ed1
Reviewed-on: https://chromium-review.googlesource.com/c/1427563
Reviewed-by: Min Qin <qinmin@chromium.org>
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Xi Han <hanxi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625191}
4 files changed