[Finishes #79456090] toggle run/reset buttons

On IE10 we weren't getting a reset button after clicking run in step mode. It turns out that what seemed to be happening was that the hidden runButton was somehow covering up our resetButton.  This fix has us disable the button in addition to hiding it, which seems to do the trick.  I also moved this code into a singular place where we toggle whether run or reset button is enabled.

I'm not sure of any great way to fix the UI tests, since I don't know of a way to actually get the pixel value at a location.

I validated this fix manually on IE 10. I plan to run the stepMode.feature on all browsers before committing.