Enable fast flat tree traversal

- Tracking bug: https://crbug.com/906494
- Explainer (WIP): http://goo.gl/UjCQnz

No web-facing behavior change.

To performance sheriffs, I expect that this CL brings the performance improvements on perf tests,
such as blink_perf/shadow_dom.

Please assign me to a bug if you observe any performance improvements or regressions other than

Bug: 906494
Change-Id: I6a473b7938a1aa71ac36a7dc776f34684b066861
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1360197
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642381}
1 file changed