- Timeline.selection is now a Timeline as well
- Allow remove the current timeline-track selection by double-clicking
  outside-the selection
- Update the timeline-track stats based on the current selection
- Simplify DOM element creation methods
- Add separate SelectionHandler class for timeline-track

Bug: v8:10644
Change-Id: I4f15d6ab4f5ec6b7330e22769472ca3074b00edd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2565130
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Sathya Gunasekaran  <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71497}