Add kNumberStringCacheRootIndex into writable_roots

Review URL: https://chromiumcodereview.appspot.com/11660028
Patch from Haitao Feng <haitao.feng@intel.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13278 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/heap.cc

@@ -2900,6 +2900,7 @@ bool Heap::RootCanBeWrittenAfterInitialization(Heap::RootListIndex root_index) {
   RootListIndex writable_roots[] = {
     kStoreBufferTopRootIndex,
     kStackLimitRootIndex,
+    kNumberStringCacheRootIndex,
     kInstanceofCacheFunctionRootIndex,
     kInstanceofCacheMapRootIndex,
     kInstanceofCacheAnswerRootIndex,