Patch series for scry-staging

series.txt

1. Refactor CSSAgent to not refcount everything, new wrappers
2. Stub out DOMTracing domain, agent, manager
3. Capture/compare/commit snapshots on backend
4. Capture screenshots, add element tracking timeline (bars), basic screenshot content view
5. Convert to having "subview" of TimelineRecordingContentView w/ own navigation bar
6. Convert to screenshot stacks on the timeline
7. Add popover previews for stacks
8. Move scrollbar of TimelineOverview out of last row
9. Selectable screenshot stacks
10. Show snapshot diff view when two snapshots selected
11. Refactor to use ContentViewGridController
12. Capture DOM tree, show in UI w/ snapshot
13. Capture effective CSS styles, show in UI w/ snapshot
14. Compute per-node change summaries, stub out UI hook
15. Show change markers in the DOM tree
16. Show change markers in the effective style view
17. Trace basic operations.
18. Stub out operation trace UI
19. Compute dependencies between DOM attr/structure operations
20. Capture animation/transition properties in the style snapshot
21. Force software rendering for elements in the tracked subtree