Port Extension Commands to Mac.

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

 {
   "name": "A browser action with no icon that makes the page red",
   "version": "1.0",
   "manifest_version": 2,
   "background": {
     "scripts": ["background.js"]
   },
   "permissions": [
     "tabs", "http://*/*", "experimental", "commands"
   ],
   "browser_action": {
     "default_title": "Make this page red",
     "default_icon": "icon.png"
   },
   "commands": {
     "toggle-feature": {
       "suggested_key": {
         "windows": "Ctrl+Shift+Y",
-        "mac": "Command+Shift+Y",
+        "mac": "Ctrl+Shift+Y",
         "linux": "Ctrl+Shift+Y",
         "chromeos": "Ctrl+Shift+Y",
         "default": "Ctrl+Shift+Y"
       },
       "description": "Toggle feature foo"
     },
     "_execute_browser_action": {
       "suggested_key": {
-        "mac": "Command+Shift+F",
         "default": "Ctrl+Shift+F"
       }
     }
   }
 }