Modify HttpResponseBodyDrainer for HttpStreamBase

Currently HttpResponseBodyDrainer requires an HttpStream object. But it
works equally well with an HttpStreamBase object, and this permits it to
be used with the new WebSocket implementation.

No functional changes.

This CL is split off from https://codereview.chromium.org/25417005/

BUG=
TEST=net_unittests

Review URL: https://codereview.chromium.org/55263004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232426 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232426