Remove unused EditorComponent::componentWillUpdate hook
Pause scroll view measurement when requesting animation frames

We don't want any extra DOM reading that could introduce hitches in
the animation we're running.
Don't defer updates with setImmediate in animation frames

I previously thought this was okay, but now I'm experiencing jitter when
scrolling with the trackpad when updates are deferred, and the frames
seem jagged. So this commit restores a synchronous approach to display
updates whenever we use animation frames.
Pause scroll view measurement in requestAnimationFrame helper method
Ignore 'scroll' events when an update is pending

This prevents feedback loops where we handle stale 'scroll' events
for scrolls requested in the model layer. It prevents jitter when
autoscrolling with the cursor.
Merge pull request #2801 from atom/ns-react-fix-jitter

Fix jitter in React editor