Deduct timeout execution time from a continuously regenerating
execution budget. Then throttle timeouts by using that budget in
TimeoutManager::MinSchedulingDelay to adjust the minimum value if
the budget is negative. The minimum value is adjusted to be a
value where the budget would have regenerated to be +0 ms.
The execution budget is clamped by values in ms defined in prefs:
* dom.timeout.background_throttling_max_budget: 50
* dom.timeout.foreground_throttling_max_budget: -1
A value equal or less than 0 means that the budget is infinite.
The regeneration rate can be controlled by the following prefs:
* dom.timeout.background_budget_regeneration_rate
* dom.timeout.foreground_budget_regeneration_rate
one each for foreground and background throttling.
To not starve timeouts indefinitely we clamp the minimum delay using
the pref:
* dom.timeout.budget_throttling_max_delay: 15000
The feature is behind the pref:
* dom.timeout.enable_budget_timer_throttling
