[heap] Start concurrent marking simultaneously with incremental marking.

src/heap/mark-compact.cc

Index: src/heap/mark-compact.cc
diff --git a/src/heap/mark-compact.cc b/src/heap/mark-compact.cc
index d89914f2fe6d4bed943d04609a5f8e6fd13ac2fb..28de72ec465bbd3e7b9798d3c198ccf7539fe4d8 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -15,6 +15,7 @@
 #include "src/gdb-jit.h"
 #include "src/global-handles.h"
 #include "src/heap/array-buffer-tracker.h"
+#include "src/heap/concurrent-marking.h"
 #include "src/heap/gc-tracer.h"
 #include "src/heap/incremental-marking.h"
 #include "src/heap/mark-compact-inl.h"
@@ -847,6 +848,8 @@ void MarkCompactCollector::Prepare() {
   // them here.
   heap()->memory_allocator()->unmapper()->WaitUntilCompleted();
 
+  heap()->concurrent_marking()->EnsureTaskCompleted();
+
   // Clear marking bits if incremental marking is aborted.
   if (was_marked_incrementally_ && heap_->ShouldAbortIncrementalMarking()) {
     heap()->incremental_marking()->Stop();