function isInputEventFromLabelClick(event) {
  return lastLabelClickPos
      && lastLabelClickPos.x === event.x
      && lastLabelClickPos.y === event.y;
}
Gets whether the given event is an input event that was caused by clicking on an
associated label element.

This is necessary because the browser will, upon clicking on a label element, fire an
*extra* click event on its associated input (if any). mdGesture is able to flag the label
click as with `$material` correctly, but not the second input click.

In order to determine whether an input event is from a label click, we compare the (x, y) for
the event to the (x, y) for the most recent label click (which is cleared whenever a non-label
click occurs). Unfortunately, there are no event properties that tie the input and the label
together (such as relatedTarget).

@param {MouseEvent} event
@returns {boolean}