Remove unused maps during marking garbage collections.

src/runtime.cc

Index: src/runtime.cc
===================================================================
--- src/runtime.cc	(revision 636)
+++ src/runtime.cc	(working copy)
@@ -320,9 +320,19 @@
 static Object* Runtime_GetTemplateField(Arguments args) {
   ASSERT(args.length() == 2);
   CONVERT_CHECKED(HeapObject, templ, args[0]);
-  RUNTIME_ASSERT(templ->IsStruct());
   CONVERT_CHECKED(Smi, field, args[1]);
-  return HeapObject::GetHeapObjectField(templ, field->value());
+  int index = field->value();
+  int offset = index * kPointerSize + HeapObject::kHeaderSize;
+  InstanceType type = templ->map()->instance_type();
+  RUNTIME_ASSERT(type ==  FUNCTION_TEMPLATE_INFO_TYPE ||
+                 type ==  OBJECT_TEMPLATE_INFO_TYPE);
+  RUNTIME_ASSERT(offset > 0);
+  if (type ==  FUNCTION_TEMPLATE_INFO_TYPE) {
+    RUNTIME_ASSERT(offset < FunctionTemplateInfo::kSize);
+  } else {
+    RUNTIME_ASSERT(offset < ObjectTemplateInfo::kSize);
+  }
+  return *HeapObject::RawField(templ, offset);
 }