Introduce --always-incremental-marking flag.

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index 4855ac08aeb8e1a56b23df4e527edd97208e977e..a3113c8177964beb2890fafc720870fedee6b45d 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -860,10 +860,11 @@ bool Heap::CollectGarbage(GarbageCollector collector,
 
   // Start incremental marking for the next cycle. The heap snapshot
   // generator needs incremental marking to stay off after it aborted.
-  if (!mark_compact_collector()->abort_incremental_marking() &&
-      incremental_marking()->IsStopped() &&
-      incremental_marking()->WorthActivating() &&
-      NextGCIsLikelyToBeFull()) {
+  if (incremental_marking()->IsStopped() &&
+      (FLAG_always_incremental_marking ||
+       (!mark_compact_collector()->abort_incremental_marking() &&
+        incremental_marking()->WorthActivating() &&
+        NextGCIsLikelyToBeFull()))) {
     incremental_marking()->Start();
   }