* 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.
I got the following error when running `checkout-head-revision`:
```
Uncaught TypeError: undefined is not a function /Applications/Atom.app/Contents/Resources/app/src/editor-component.js:804
addCommandListeners.editor:checkout-head-revision /Applications/Atom.app/Contents/Resources/app/src/editor-component.js:804
(anonymous function) /Applications/Atom.app/Contents/Resources/app/src/editor-component.js:840
jQuery.event.dispatch /Applications/Atom.app/Contents/Resources/app/node_modules/space-pen/vendor/jquery.js:4676
elemData.handle /Applications/Atom.app/Contents/Resources/app/node_modules/space-pen/vendor/jquery.js:4360
module.exports.KeymapManager.dispatchCommandEvent /Applications/Atom.app/Contents/Resources/app/node_modules/atom-keymap/lib/keymap-manager.js:400
module.exports.KeymapManager.handleKeyboardEvent /Applications/Atom.app/Contents/Resources/app/node_modules/atom-keymap/lib/keymap-manager.js:181
module.exports.WindowEventHandler.onKeydown
```
It looks like it was caused by a bad merge conflict resolution in 0eaec57.
Test Plan:
Built new Atom and pressed `cmd-alt-z` and didn't get an error.
Looked at a side-by-side diff to check for any other merge issues:
```
git difftool -t opendiff 32e59ce..32e59ce^ src/editor-component.coffee
```
