Don't create debug context if debug listener is not set

src/runtime/runtime-debug.cc

 // Copyright 2014 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.
 
 #include "src/v8.h"
 
 #include "src/accessors.h"
 #include "src/arguments.h"
 #include "src/compiler.h"
 #include "src/debug.h"
@@ skipping 2815 matching lines @@
       isolate, result,
       Execution::Call(isolate, function, handle(function->global_proxy()), 0,
                       NULL));
   return *result;
 }
 
 
 RUNTIME_FUNCTION(Runtime_GetDebugContext) {
   HandleScope scope(isolate);
   DCHECK(args.length() == 0);
-  Handle<Context> context = isolate->debug()->GetDebugContext();
+  Handle<Context> context;
+  {
+    DebugScope debug_scope(isolate->debug());
+    context = isolate->debug()->GetDebugContext();
+  }
   if (context.is_null()) return isolate->heap()->undefined_value();
   context->set_security_token(isolate->native_context()->security_token());
   return context->global_proxy();
 }
 
 
 // Performs a GC.
 // Presently, it only does a full GC.
 RUNTIME_FUNCTION(Runtime_CollectGarbage) {
   SealHandleScope shs(isolate);
@@ skipping 134 matching lines @@
   return Smi::FromInt(isolate->debug()->is_active());
 }
 
 
 RUNTIME_FUNCTION(Runtime_DebugBreakInOptimizedCode) {
   UNIMPLEMENTED();
   return NULL;
 }
 }
 }  // namespace v8::internal