Allow incremental marking when expose_gc is turned on.

src/incremental-marking.cc

Index: src/incremental-marking.cc
diff --git a/src/incremental-marking.cc b/src/incremental-marking.cc
index e420d28521ec26de2f7a01151e3ade68a28c6235..8cdcdb5e5e4364102baee95166d7424fa7a503dc 100644
--- a/src/incremental-marking.cc
+++ b/src/incremental-marking.cc
@@ -493,8 +493,7 @@ bool IncrementalMarking::WorthActivating() {
   static const intptr_t kActivationThreshold = 0;
 #endif
 
-  return !FLAG_expose_gc &&
-      FLAG_incremental_marking &&
+  return FLAG_incremental_marking &&
       !Serializer::enabled() &&
       heap_->PromotedSpaceSizeOfObjects() > kActivationThreshold;
 }