Layout: minor optimizations and bugfix for non-functional details (#38197)

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.

---------

Signed-off-by: sharpshooter_pt <ibluegalaxy_taoj@163.com>

components/layout/flexbox/layout.rs

@@ -1179,7 +1179,11 @@ fn do_initial_flex_line_layout<'items>(
     // We didn't reach the end of the last line, so add all remaining items there.
     lines.push((items, line_size_so_far));
 
-    lines.par_drain(..).map(construct_line).collect()
+    if flex_context.layout_context.use_rayon {
+        lines.par_drain(..).map(construct_line).collect()
+    } else {
+        lines.drain(..).map(construct_line).collect()
+    }
 }
 
 /// The result of splitting the flex items into lines using their intrinsic sizes and doing an