bug 1058722 - Add ability to filter stack when invoking the task manager...

.... r=sfoster
Nit: Seems like this extra slice is redundant here? 

unfiltered = StackManager.snapshot(); 
if (typeof filter === 'function') {
  stack = this.stack = unfiltered.filter(filer); 
  stack.unfiltered = filter;
} else {
  stack = this.stack = unfiltered; 
}