h2: Notify delegate about read EOF

So the delegate can close the socket instead of keeping sending data.

Read EOF or h2 half-closed (remote) state was introduced in
https://codereview.chromium.org/129543002. But StreamSocket doesnt
really supports a half closed state, so upon a read EOF the only sane
action is to close the socket immediately even if in theory more send
is possible.

net/spdy/spdy_stream.cc

@@ -538,6 +538,7 @@ void SpdyStream::OnDataReceived(std::unique_ptr<SpdyBuffer> buffer) {
   if (!buffer) {
     if (io_state_ == STATE_OPEN) {
       io_state_ = STATE_HALF_CLOSED_REMOTE;
+      delegate_->OnDataReceived(nullptr);
     } else if (io_state_ == STATE_HALF_CLOSED_LOCAL) {
       io_state_ = STATE_CLOSED;
       // Deletes |this|.