[debug,api] Do not use embedder field for debug context id.

src/debug/debug-interface.h

 // Copyright 2016 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_DEBUG_INTERFACE_H_
 #define V8_DEBUG_DEBUG_INTERFACE_H_
 
 #include <functional>
 
 #include "include/v8-debug.h"
 #include "include/v8-util.h"
 #include "include/v8.h"
 
 #include "src/debug/interface-types.h"
 #include "src/globals.h"
 
 namespace v8 {
 
 namespace internal {
 struct CoverageFunction;
 struct CoverageScript;
 class Coverage;
 class Script;
 }
 
 namespace debug {
 
+void SetContextId(Local<Context> context, int id);
+int GetContextId(Local<Context> context);
+
 /**
  * Debugger is running in its own context which is entered while debugger
  * messages are being dispatched. This is an explicit getter for this
  * debugger context. Note that the content of the debugger context is subject
  * to change. The Context exists only when the debugger is active, i.e. at
  * least one DebugEventListener or MessageHandler is set.
  */
 Local<Context> GetDebugContext(Isolate* isolate);
 
 /**
@@ skipping 244 matching lines @@
   ~Coverage();
 
  private:
   explicit Coverage(i::Coverage* coverage) : coverage_(coverage) {}
   i::Coverage* coverage_;
 };
 }  // namespace debug
 }  // namespace v8
 
 #endif  // V8_DEBUG_DEBUG_INTERFACE_H_