Wait for sweeper threads in incremental marking step when sweeper threads are done sweeping.

src/incremental-marking.cc

Index: src/incremental-marking.cc
diff --git a/src/incremental-marking.cc b/src/incremental-marking.cc
index c32155e1430450875e653de2a68bf3411da21364..bb2c9ac2388b26a74f190fade21a364369acf276 100644
--- a/src/incremental-marking.cc
+++ b/src/incremental-marking.cc
@@ -886,6 +886,10 @@ void IncrementalMarking::Step(intptr_t allocated_bytes,
   }
 
   if (state_ == SWEEPING) {
+    if (heap_->mark_compact_collector()->IsConcurrentSweepingInProgress() &&
+        heap_->mark_compact_collector()->IsSweepingCompleted()) {
+      heap_->mark_compact_collector()->WaitUntilSweepingCompleted();
+    }
     if (!heap_->mark_compact_collector()->IsConcurrentSweepingInProgress()) {
       bytes_scanned_ = 0;
       StartMarking(PREVENT_COMPACTION);