[UMA] Adds start/finish NativeBackgroundTask metrics

There are no metrics being recorded for clean task completion (ie. no
chrome crashes or OOMs during task running). This cl adds new
start/finish metrics to native background task in order to get a
percentage of tasks that fail unexpectedly.

Bug: 924644
Change-Id: Ic1a70ff5aad5aca633c5e50ed32b1dc53b60120f
Reviewed-on: https://chromium-review.googlesource.com/c/1428092
Commit-Queue: Mohamed Heikal <mheikal@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Xi Han <hanxi@chromium.org>
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Auto-Submit: Mohamed Heikal <mheikal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626239}
5 files changed