Don't execute the fallback if we already started running the gc callback

extensions/renderer/gc_callback.cc

 // Copyright 2015 The Chromium 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 "extensions/renderer/gc_callback.h"
 
 #include "base/bind.h"
 #include "base/message_loop/message_loop.h"
 #include "extensions/renderer/script_context.h"
 
@@ skipping 23 matching lines @@
   // DO NOT add any more work to this method. The only acceptable place to add
   // code is RunCallback.
   GCCallback* self = data.GetParameter();
   self->object_.Reset();
   base::MessageLoop::current()->PostTask(
       FROM_HERE, base::Bind(&GCCallback::RunCallback,
                             self->weak_ptr_factory_.GetWeakPtr()));
 }
 
 void GCCallback::RunCallback() {
+  fallback_.Reset();
   v8::Isolate* isolate = context_->isolate();
   v8::HandleScope handle_scope(isolate);
   context_->CallFunction(v8::Local<v8::Function>::New(isolate, callback_));
   delete this;
 }
 
 void GCCallback::OnContextInvalidated() {
-  fallback_.Run();
-  delete this;
+  if (!fallback_.is_null()) {
+    fallback_.Run();
+    delete this;
+  }
 }
 
 }  // namespace extensions