misc/chrome/gophertool: Support for manifest version 1 is being phased out.

Manifest version 1 was deprecated in Chrome 18, and support will be phased
out according to the schedule available at:
http://code.google.com/chrome/extensions/manifestVersion.html

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6332053

misc/chrome/gophertool/manifest.json

 {
   "name": "Hacking Gopher",
   "version": "1.0",
+  "manifest_version": 2,
   "description": "Go Hacking utility",
-  "background_page": "background.html",
+  "background": {
+    "page": "background.html"
+  },
   "browser_action": {
     "default_icon": "gopher.png",
     "popup": "popup.html"