* refactor: desttroy URLRequestContextGetter on IO thread
* Accepts a factory class that can customize the creation of URLRequestContext
* Use a separate request context for media which is derived from the default
* Notify URLRequestContextGetter observers and cleanup on IO thread
* Move most of brightray net/ classes into atom net/
* refactor: remove refs to URLRequestContextGetter on shutdown
* refactor: remove brigtray switches.{cc|h}
* refactor: remove brightray network_delegate.{cc|h}
* refactor: make AtomURLRequestJobFactory the top level factory.
* Allows to use the default handler from content/ for http{s}, ws{s} schemes.
* Removes the storage of job factory in URLRequestContextGetter.
