using content::BrowserThread
Give user a chance to create custom URLRequestJobFactory.
Clean handlers when custom job factory is used.
Add url_request_context_getter getter for BrowserContext.
Pass ProtocolHandlerScopedVector by value.
Pass parameter by pointers when creating job factory.

1. The caller needs to modify protocol_handlers.
2. We need to make sure protocol_handlers and interceptors are not touched
   if user doesn't create a job factory.
Remove unneeded headers and declarations.
Merge pull request #68 from brightray/expose-url_request_context_getter

Add CreateURLRequestJobFactory override for BrowserContext