Fix concurrent sweeping in predictable mode and bring --concurrent-sweeping flag back.

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 908b686c8813bf3c3ab4cadc18978919f9729ac6..a206b1d12e4beb525a4ffb2235e37ee9571ec830 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -470,12 +470,12 @@ void MarkCompactCollector::EnsureSweepingCompleted() {
 
   // If sweeping is not completed or not running at all, we try to complete it
   // here.
-  if (FLAG_predictable || !IsSweepingCompleted()) {
+  if (!FLAG_concurrent_sweeping || !IsSweepingCompleted()) {
     SweepInParallel(heap()->paged_space(OLD_DATA_SPACE), 0);
     SweepInParallel(heap()->paged_space(OLD_POINTER_SPACE), 0);
   }
   // Wait twice for both jobs.
-  if (!FLAG_predictable) {
+  if (FLAG_concurrent_sweeping) {
     pending_sweeper_jobs_semaphore_.Wait();
     pending_sweeper_jobs_semaphore_.Wait();
   }
@@ -4145,7 +4145,7 @@ void MarkCompactCollector::SweepSpaces() {
       SweepSpace(heap()->old_data_space(), CONCURRENT_SWEEPING);
     }
     sweeping_in_progress_ = true;
-    if (!FLAG_predictable) {
+    if (FLAG_concurrent_sweeping) {
       StartSweeperThreads();
     }
   }