Add preferences support for embedding applications

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
Remember where the dev tools were docked between launches