The context: https://crbug.com/699838.

updateLayoutTree tends to be slower in case 2 (with Shadow DOM)

- updateLayoutTree (This is *max*, neither sum nor avg)
  - app1:  33.189ms
  - app2:  81.728ms

Note that updateLayoutTree happens 4 times in each case, but only the most significant one is important here.

[1]: https://bugs.chromium.org/p/chromium/issues/detail?id=699838#c8

This result is not acceptable, I think. 

UpdateLayoutTree with a Shadow DOM should be as fast as that without Shadow DOM, or should be faster.

Let me break down the trace.