Turn the NumberToStringStub into a hydrogen stub.

src/hydrogen.h

Index: src/hydrogen.h
diff --git a/src/hydrogen.h b/src/hydrogen.h
index a371fa571b8e3e370a495e4f19dba88f2f916d89..35433bd3856cc815b26109dc70d361f48eed9bb5 100644
--- a/src/hydrogen.h
+++ b/src/hydrogen.h
@@ -1237,6 +1237,15 @@ class HGraphBuilder {
                                    ElementsKind to_kind,
                                    bool is_jsarray);
 
+  // Do lookup in the number string cache. If the object is not found
+  // in the cache, the false branch of the continuation is taken;
+  // otherwise the true branch is taken and the returned value contains
+  // the cache value for the object. The returned value must NOT be used
+  // on the false branch.
+  HValue* BuildLookupNumberStringCache(HValue* object,
+                                       HIfContinuation* continuation);
+  HValue* BuildNumberToString(HValue* number);
+
   HInstruction* BuildUncheckedMonomorphicElementAccess(
       HValue* checked_object,
       HValue* key,