Introduce HttpStream and HttpBasicStream.
This is the beginning of the http pipelining work. Introduce HttpStream, an interface for reading and writing to http streams. Provide a basic implementation with HttpBasicStream. Switch HttpNetworkTransaction to reading/writing via HttpStream rather than directly to the socket.
Note that the interface will have to change later on. Read/Write() is the wrong interface, since a pipelining HttpStream implementation will have to detect the end of an http response, rather than having the client (HttpNetworkTransaction) do that. This is just the first step.
For information of the general roadmap for http pipelining, please refer to the bug.