[Isolates] Add debugging code to ensure consistent Isolate class layout.

src/isolate.cc

 // Copyright 2006-2010 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 783 matching lines @@
 
   // Reinit the current thread for the isolate it was running before this one.
   SetIsolateThreadLocals(previous_isolate, previous_thread_data);
 }
 
 
 void Isolate::ResetEagerOptimizingData() {
   compilation_cache_->ResetEagerOptimizingData();
 }
 
+
+#ifdef DEBUG
+#define ISOLATE_FIELD_OFFSET(type, name, ignored)                       \
+const intptr_t Isolate::name##_debug_offset_ = OFFSET_OF(Isolate, name##_);
+ISOLATE_INIT_LIST(ISOLATE_FIELD_OFFSET)
+ISOLATE_INIT_ARRAY_LIST(ISOLATE_FIELD_OFFSET)
+#undef ISOLATE_FIELD_OFFSET
+#endif
+
 } }  // namespace v8::internal