MIPS: Only extend storage if FIELD

Port r15125 (0c202ff0)

BUG=

Review URL: https://codereview.chromium.org/16983002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15134 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/mips/stub-cache-mips.cc

@@ -537,7 +537,8 @@ void StubCompiler::GenerateStoreTransition(MacroAssembler* masm,
   ASSERT(object->IsJSGlobalProxy() || !object->IsAccessCheckNeeded());
 
   // Perform map transition for the receiver if necessary.
-  if (object->map()->unused_property_fields() == 0) {
+  if (details.type() == FIELD &&
+      object->map()->unused_property_fields() == 0) {
     // The properties must be extended before we can store the value.
     // We jump to a runtime call that extends the properties array.
     __ push(receiver_reg);