[heap] Cleanup and fix GC flags / add testing infra

src/heap/incremental-marking.cc

Index: src/heap/incremental-marking.cc
diff --git a/src/heap/incremental-marking.cc b/src/heap/incremental-marking.cc
index 544ee7458baf075324f7f5220506171bfc44289f..9549a148b488362b456b1d99fc9f4b7775704071 100644
--- a/src/heap/incremental-marking.cc
+++ b/src/heap/incremental-marking.cc
@@ -468,7 +468,7 @@ static void PatchIncrementalMarkingRecordWriteStubs(
 }
 
 
-void IncrementalMarking::Start(int mark_compact_flags,
+void IncrementalMarking::Start(int flags,
                                const GCCallbackFlags gc_callback_flags,
                                const char* reason) {
   if (FLAG_trace_incremental_marking) {
@@ -486,9 +486,8 @@ void IncrementalMarking::Start(int mark_compact_flags,
   was_activated_ = true;
 
   if (!heap_->mark_compact_collector()->sweeping_in_progress()) {
-    heap_->mark_compact_collector()->SetFlags(mark_compact_flags);
+    heap_->set_current_gc_flags(flags);
     StartMarking();
-    heap_->mark_compact_collector()->SetFlags(Heap::kNoGCFlags);
   } else {
     if (FLAG_trace_incremental_marking) {
       PrintF("[IncrementalMarking] Start sweeping.\n");