Implement handlified String::Flatten.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index a9546974a752314a585b0b54ebc05d899a13d225..c17faff12930ac95c192c4f26733b3a4c9b7cb5f 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -9005,6 +9005,13 @@ class String: public Name {
   // string.
   inline String* TryFlattenGetString(PretenureFlag pretenure = NOT_TENURED);
 
+  static inline Handle<String> Flatten(Handle<String> string,
+                                       PretenureFlag pretenure = NOT_TENURED);
+
+  static Handle<String> SlowFlatten(Handle<ConsString> cons,
+                                    PretenureFlag tenure);
+
+
   // Tries to return the content of a flat string as a structure holding either
   // a flat vector of char or of uc16.
   // If the string isn't flat, and therefore doesn't have flat content, the