TryFlatten is inlined, while Flatten is not. Make an optimization to avoid t...

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 271dd7ebf9432da075cf63dc716172efc1c29197..8fec4d1630e6a7c478eff0ebb3956805fb51ca7f 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -1198,7 +1198,11 @@ void String::set_length_field(int value) {
 
 
 void String::TryFlatten() {
-  Flatten();
+  // We don't need to flatten strings that are already flat.  Since this code
+  // is inlined, it can be helpful in the flat case to not call out to Flatten.
+  StringRepresentationTag str_type = representation_tag();
+  if (str_type != kSeqStringTag && str_type != kExternalStringTag)
+    Flatten();
 }