Bug 857648 part 1.  Remove unnecessary code that tries to filter out non-JS stack frames from a known-JS-only stack.  r=khuey
Bug 857648 part 2.  Split JSStackFrame into a generic class and a JS-specific class.  r=khuey
Bug 857648 part 3.  Switch from using JS::DescribeStack to JS::CaptureCurrentStack for producing JSStackFrames.  r=khuey
Bug 857648 part 4.  Add a formattedStack attribute on nsIStackFrame.  r=khuey
Bug 857648 part 5.  Expose a .stack property on DOMExceptions.  r=khuey
Bug 857648 part 6.  Remove the now-unused JS::DescribeStack API.  r=jimb