Polish the keybinding implementation a bit.

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", "keybinding"
   ],
   "browser_action": {
     "default_title": "Make this page red",
     "default_icon": "icon.png"
   },
   "commands": {
     "toggle-feature": {
-      "key": "  Ctrl - Shift  - Y  ",
+      "suggested_key": {
+        "windows": "Ctrl+Shift+Y",
+        "mac": "Command+Shift+Y",
+        "linux": "Ctrl+Shift+Y",
+        "chromeos": "Ctrl+Shift+Y",
+        "default": "Ctrl+Shift+Y"
+      },
       "description": "Toggle feature foo"
     },
-    "browserAction": {
-      "key": "Ctrl+Shift+F",
-      "description": ""
+    "_execute_browser_action": {
+      "suggested_key": {
+        "mac": "Command+Shift+F",
+        "default": "Ctrl+Shift+F"
+      }
     }
   }
 }