Applications can use brightray::BrowserContext::prefs to get access to
preferences that are persisted between launches.
brightray::BrowserContext::RegisterPrefs must be used to register preferences
before they're accessed.
Updated libchromiumcontent to pull in preferences support.
* vendor/libchromiumcontent 3944c1c...b6a0d85 (2):
> Actually compile the base/prefs code
> Export symbols from base/prefs
