Make sure manifest specified shortcut for Extension Command can not override the built-in shortcuts.

chrome/browser/ui/gtk/accelerator_utils_gtk.cc

+// Copyright (c) 2013 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 "chrome/browser/profiles/profile.h"
+#include "chrome/browser/ui/gtk/accelerators_gtk.h"
+#include "ui/base/accelerators/accelerator.h"
+
+namespace chrome {
+
+bool IsChromeAccelerator(const ui::Accelerator& accelerator, Profile* profile) {
+  AcceleratorsGtk* accelerators = AcceleratorsGtk::GetInstance();
+  for (AcceleratorsGtk::const_iterator iter = accelerators->begin();
+       iter != accelerators->end(); ++iter) {
+    if (iter->second.key_code() == accelerator.key_code() &&
+        iter->second.modifiers() == accelerator.modifiers())
+      return true;
+  }
+
+  return false;
+}
+
+}  // namespace chrome