Remove sequential sweeping mode and perform lazy sweeping when no sweeper threads are active.

src/incremental-marking.cc

Index: src/incremental-marking.cc
diff --git a/src/incremental-marking.cc b/src/incremental-marking.cc
index 8e2eb62357664d39f378530af1408e0f6202b9b9..b80fb8e48e6ebc873cfcb31cabc451ef8b6bf838 100644
--- a/src/incremental-marking.cc
+++ b/src/incremental-marking.cc
@@ -536,7 +536,7 @@ void IncrementalMarking::Start(CompactionFlag flag) {
 
   ResetStepCounters();
 
-  if (!heap_->mark_compact_collector()->IsConcurrentSweepingInProgress()) {
+  if (!heap_->mark_compact_collector()->sweeping_in_progress()) {
     StartMarking(flag);
   } else {
     if (FLAG_trace_incremental_marking) {
@@ -883,11 +883,11 @@ void IncrementalMarking::Step(intptr_t allocated_bytes,
   }
 
   if (state_ == SWEEPING) {
-    if (heap_->mark_compact_collector()->IsConcurrentSweepingInProgress() &&
+    if (heap_->mark_compact_collector()->sweeping_in_progress() &&
         heap_->mark_compact_collector()->IsSweepingCompleted()) {
-      heap_->mark_compact_collector()->WaitUntilSweepingCompleted();
+      heap_->mark_compact_collector()->EnsureSweepingCompleted();
     }
-    if (!heap_->mark_compact_collector()->IsConcurrentSweepingInProgress()) {
+    if (!heap_->mark_compact_collector()->sweeping_in_progress()) {
       bytes_scanned_ = 0;
       StartMarking(PREVENT_COMPACTION);
     }