Set user agent string to Chrome

Issue: tiliado/nuvolaplayer#91

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>

metadata.json

@@ -10,5 +10,6 @@
     "categories": "AudioVideo;Audio;",
     "license": "2-Clause BSD-license",
     "home_url": "https://play.google.com/music/",
+    "user_agent": "CHROME",
     "allowed_uri": "^https?://(ssl\\.gstatic\\.com/|play\\.google\\.com/((?!store).*)$|accounts\\.google(\\.\\w+)+/|(www\\.)?google(\\.\\w+)+/accounts|accounts\\.youtube\\.com/accounts|.*\\.googleusercontent\\.com)"
 }