Rework DOM measurement to try to prevent measurement errors

* Simplify scrollbar refresh and measurement by using imperative DOM
  manipulation instead of React to hide/show scrollbars.
* Rename `::performInitialMeasurement` to `::becameVisible`
* Break `::checkForVisibilityChange` out of `::pollDOM` and use it in
  to check for the element becoming visible in `componentWillUpdate`.
* Don't rely on stored visibility state anywhere. Always check again.
  This could potentially be cached for an update cycle but being wrong
  about this is disastrous so I'm being conservative.
Check for visibility change when mounting instead of polling DOM
Don't use guard clauses in short measurement methods
Don't remeasure char widths until we've performed initial measurement
Merge pull request #3353 from atom/ns-fix-char-width-measurement

Rework DOM measurement to try to prevent measurement errors