Improve Chrome App template.

- index.html -> window.html
- main.js -> window.js
- Don't set window size.
- Don't set CSS, only provide the template file.
For now: 
const int kDefaultWidth = 512;
const int kDefaultHeight = 384;

https://code.google.com/p/chromium/codesearch#chromium/src/apps/app_window.cc&sq=package:chromium&q=file:app_window%20kDefaultWidth%20kDefaultHeight&type=cs&l=67