Fix double to integer conversions in runtime string indexing.

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index 80e67cc87585ce206e5ff200e0ea3a53c1240534..50a29c24f9770d31d8258cda9cf782bf722daada 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -1665,7 +1665,7 @@ static Object* Runtime_StringCharCodeAt(Arguments args) {
   } else {
     ASSERT(index->IsHeapNumber());
     double value = HeapNumber::cast(index)->value();
-    i = static_cast<uint32_t>(value);
+    i = static_cast<uint32_t>(DoubleToInteger(value));
   }
 
   // Flatten the string.  If someone wants to get a char at an index