* 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.
When intercepting HTTP protocols Chromium will assume there is always headers
set, so we have to provide headers for all the responses to avoid the
crash.
