misc/chrome/gophertool: fix manifest

according to http://code.google.com/chrome/extensions/browserAction.html,
it should use "default_popup" instead of "popup".

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/6349077

misc/chrome/gophertool/manifest.json

@@ -8,7 +8,7 @@
   },
   "browser_action": {
     "default_icon": "gopher.png",
-    "popup": "popup.html"
+    "default_popup": "popup.html"
   },
   "omnibox": { "keyword": "golang" },
   "icons": {