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

chrome/browser/ui/views/frame/browser_view.cc

Index: chrome/browser/ui/views/frame/browser_view.cc
diff --git a/chrome/browser/ui/views/frame/browser_view.cc b/chrome/browser/ui/views/frame/browser_view.cc
index 732d71a5721e554e9bafa755de7aba9525b6dc05..11a71f5a4f1518dcb648339cdd7fb7cda3a1b221 100644
--- a/chrome/browser/ui/views/frame/browser_view.cc
+++ b/chrome/browser/ui/views/frame/browser_view.cc
@@ -638,6 +638,10 @@ bool BrowserView::GetAccelerator(int cmd_id, ui::Accelerator* accelerator) {
       cmd_id, browser_->host_desktop_type(), accelerator);
 }
 
+bool BrowserView::IsAcceleratorRegistered(const ui::Accelerator& accelerator) {
+  return accelerator_table_.find(accelerator) != accelerator_table_.end();
+}
+
 WebContents* BrowserView::GetActiveWebContents() const {
   return browser_->tab_strip_model()->GetActiveWebContents();
 }