Don't call nextTick within requestAnimationFrame

This seems to be causing stutter when scrolling on the GPU. When I don't
use nextTick when requesting animation frames, the problem seems to go
away. Maybe there's some issue with the integration between the Node
and Chromium event loops plus sending things to the compositor.

/cc @benogle
Remove nextTick in spec
Properly disable nextTick when calling requestAnimationFrame

My previous commit was nonsense because it set the ::performSyncUpdates
flag back to false before the animation frame callback ever got fired.

/cc again @benogle
Make lines 100% of the width of their container

This is part of the solution to #2701. Lines need to be 100% wide in
case a class styles their background.