This change adds some minor optimizations and bugfix for non-functional
details with seperated commits:
- fix the omission that stop use `Rayon` in single-thread mode
- add trace for incremental box tree construction
- fix the bug that failed to skip reflow entirely when there is no need
for `restyle` and a fragment tree has already been built.
- add trace for stylist preparation during reflow. In certain scenarios,
this phase might take up a significant amount of time, such as when
there are a large number of shadow trees.
Testing: This should not change observable behavior and is thus covered
by existing WPT tests.
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Signed-off-by: sharpshooter_pt <ibluegalaxy_taoj@163.com>
8a1cc69717