FIX: restore b2g agent sheet scrollbars on b2g windows

using code based on FloatingScrollbars module from devtools:
http://mxr.mozilla.org/mozilla-central/source/browser/devtools/shared/FloatingScrollbars.jsm

adapted to our usecase:
- app iframe and browser app tabs detected by mutation observer
- inject full original b2g agent sheet css instead of the floating-scrollbars.css
  (e.g. to style buttons as in the FirefoxOS device)

NOTE: there're some remaining issues related to shell styling, 
because currently any try to inject b2g agent sheet into the homescreen
iframe generates some weird side-effect (homescreen change rotation
instead to lock to portrait) but probably it's more important to restore the style for apps now, and then in a second time try to restore it on the homescreen as well.