Reduce Jankometer measurements by only periodically sampling
Reduce Jankometer measurements by only periodically sampling

The Jankometer is monitoring durations for each
task, but most of the time, this work is not used.
I changed the system to only sample the durations,
unless someone turns on a watchdog, and then we
monitor every event.  The sampling is deterministic,
(every N events is sampled) but should be fine 
when we aggregate across thousands of users in
a histogram.

r=brettw
Review URL: http://codereview.chromium.org/4279003

git-svn-id: http://src.chromium.org/svn/trunk/src@64991 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 9d1e1129ece0988447eae71b43027d548542e4ce
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser