[heap] Trace MajorGC's initial root marking using MC_MARK_ROOTS event

Previously, only root marking performed during the final pause
was accounted for in the tracing data.

This CL enables tracing of the initial root marking step of MajorMC.

Change-Id: I4aa8a52144d81a12e43a481518acbab118978992
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3724793
Commit-Queue: Leon Bettscheider <bettscheider@google.com>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81378}

src/heap/incremental-marking.cc

@@ -279,7 +279,10 @@ void IncrementalMarking::StartMarking() {
 
   StartBlackAllocation();
 
-  MarkRoots(heap_);
+  {
+    TRACE_GC(heap()->tracer(), GCTracer::Scope::MC_MARK_ROOTS);
+    MarkRoots(heap_);
+  }
 
   if (FLAG_concurrent_marking && !heap_->IsTearingDown()) {
     heap_->concurrent_marking()->ScheduleJob();