Bug 932837 part 1.  Make FrameDescription compute the line number lazily.  r=jandem
Bug 932837 part 2.  Create a refcounted object to manage the lifetime of a JS::StackDescription.  r=mccr8, terrence
Bug 932837 part 3.  Make JSStackFrame get information from the JS stack lazily.  r=khuey
Bug 932837 part 4.  Allocate the caller JSStackFrames lazily.  r=khuey
Bug 932837 part 5.  Don't eagerly grab file/line info when constructing an Exception.  r=khuey