Make sweeper threads respect the max_available_threads constraint.

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 16decbb39cacaccb4c21d533cfe6dbc9671f345c..f70cab1860559402262261c62f9870d3e3272122 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -472,12 +472,12 @@ void MarkCompactCollector::EnsureSweepingCompleted() {
 
   // If sweeping is not completed or not running at all, we try to complete it
   // here.
-  if (!FLAG_concurrent_sweeping || !IsSweepingCompleted()) {
+  if (!heap()->concurrent_sweeping_enabled() || !IsSweepingCompleted()) {
     SweepInParallel(heap()->paged_space(OLD_DATA_SPACE), 0);
     SweepInParallel(heap()->paged_space(OLD_POINTER_SPACE), 0);
   }
   // Wait twice for both jobs.
-  if (FLAG_concurrent_sweeping) {
+  if (heap()->concurrent_sweeping_enabled()) {
     pending_sweeper_jobs_semaphore_.Wait();
     pending_sweeper_jobs_semaphore_.Wait();
   }
@@ -4238,7 +4238,7 @@ void MarkCompactCollector::SweepSpaces() {
       SweepSpace(heap()->old_data_space(), CONCURRENT_SWEEPING);
     }
     sweeping_in_progress_ = true;
-    if (FLAG_concurrent_sweeping) {
+    if (heap()->concurrent_sweeping_enabled()) {
       StartSweeperThreads();
     }
   }