Remove HttpStreamBase.

HttpStreamBase was introduced to integrate the connection setup logic between
HTTP and WebSocket[1] by abstracting methods from HttpStream, but it is not
needed now. In addition to that, there is a downcast from HttpStreamBase to
HttpStream and that leads to a crash.

This CL moves all methods in HttpStreamBase to HttpStream and removes
HttpStreamBase.
This CL also replaces NULL with nullptr in some files.

1: https://chromium.googlesource.com/chromium/src/+/92b9a3e000136518c489975bebea57bc0f66333f
BUG=399535

Review URL: https://codereview.chromium.org/699123002

Cr-Commit-Position: refs/heads/master@{#302967}
17 files changed