Remove the dependency of Zone on Isolate

test/cctest/cctest.h

 // Copyright 2008 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 559 matching lines @@
   i::Isolate* main_isolate() { return main_isolate_; }
 
  private:
   i::Isolate* main_isolate_;
   i::HandleScope handle_scope_;
 };
 
 
 class HandleAndZoneScope : public InitializedHandleScope {
  public:
-  HandleAndZoneScope() : main_zone_(main_isolate()) {}
+  HandleAndZoneScope() {}
 
   // Prefixing the below with main_ reduces a lot of naming clashes.
   i::Zone* main_zone() { return &main_zone_; }
 
  private:
   i::Zone main_zone_;
 };
 
 #endif  // ifndef CCTEST_H_