samples/pop_pop_win: enable difficulty in chrome app

[kevmoo]

From @kevmoo on May 29, 2014 18:36

The web version of PPW uses URL fragments to control difficulty. This is not possible in a chrome app

See samples/pop_pop_win/chrome_app/index.html

To test: pub build chrome_app then load the unpackaged app in build/chrome_app in Chrome

Copied from original issue: dart-lang/sdk#19087