Introduce phantom weak handles in the API and use them internally for debug info

src/debug.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_DEBUG_H_
 #define V8_DEBUG_H_
 
 #include "src/allocation.h"
 #include "src/arguments.h"
 #include "src/assembler.h"
@@ skipping 541 matching lines @@
                             Handle<JSObject> event_data,
                             bool auto_continue);
   void InvokeMessageHandler(MessageImpl message);
 
   static bool CompileDebuggerScript(Isolate* isolate, int index);
   void ClearOneShot();
   void ActivateStepIn(StackFrame* frame);
   void ClearStepIn();
   void ActivateStepOut(StackFrame* frame);
   void ClearStepNext();
-  // Returns whether the compile succeeded.
-  void RemoveDebugInfo(Handle<DebugInfo> debug_info);
+  void RemoveDebugInfoAndClearFromShared(Handle<DebugInfo> debug_info);
+  void RemoveDebugInfo(DebugInfo** debug_info);
   Handle<Object> CheckBreakPoints(Handle<Object> break_point);
   bool CheckBreakPoint(Handle<Object> break_point_object);
 
   inline void AssertDebugContext() {
     DCHECK(isolate_->context() == *debug_context());
     DCHECK(in_debug_scope());
   }
 
   void ThreadInit();
 
@@ skipping 172 matching lines @@
   // several frames above.
   // There is no calling conventions here, because it never actually gets
   // called, it only gets returned to.
   static void GenerateFrameDropperLiveEdit(MacroAssembler* masm);
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_DEBUG_H_