Allow 4 more fast properties for objects.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index b35916a3369802bbef6cbbbb947e2b45dab5adcc..c5fda7d0385d10d299754204a1c79c9d8b3ef33a 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -1734,7 +1734,7 @@ class JSObject: public HeapObject {
   static const uint32_t kMaxGap = 1024;
   static const int kMaxFastElementsLength = 5000;
   static const int kInitialMaxFastElementArray = 100000;
-  static const int kMaxFastProperties = 8;
+  static const int kMaxFastProperties = 12;
   static const int kMaxInstanceSize = 255 * kPointerSize;
   // When extending the backing storage for property values, we increase
   // its size by more than the 1 entry necessary, so sequentially adding fields