Allow extensions to remove and override the bookmark shortcut key

chrome/test/data/extensions/api_test/keybinding/dont_overwrite_system/manifest.json

 {
   "name": "An extension that tries to overwrite system keybindings",
   "version": "1.0",
   "manifest_version": 2,
   "background": {
     "scripts": ["background.js"]
   },
   "permissions": ["activeTab"],
   "commands": {
     "red": {
       "suggested_key": "Ctrl+F",
       "description": "Make the page red"
     },
+    "green": {
+      "suggested_key": "Ctrl+D",
+      "description": "Make the page green"
+    },
     "blue": {
       "suggested_key": "Alt+Shift+F",
       "description": "Make the page blue"
     }
   }
 }