Adds a PeformanceCounter class that is used in DocGroup and WorkerPrivate
to track runnables execution and dispatch counts.
MozReview-Commit-ID: 51DLj6ORD2O
The canvas prompt is extremely annoying. It happens everyone, automatically. And in
99.9% (not scientific) of cases it is not triggered by user input, but my automatic
tracking scripts.
This commit will automatically decline the canvas read if it was not triggered by
user input.
Just in case this breaks something irrepairably, we have a cutoff pref.
We don't intend to keep this pref forever, and have asked anyone who sets it to
tell us why.
MozReview-Commit-ID: CxNkuraRWpV
