gfe-relnote: Add NumRegisteredStreams interface to WriteScheduler and implements it in subclasses. No used in prod yet, not protected.

Also move WriteSchedulerType to third_party/spdy/spdy_protocol.h to allow it be used by QUIC.

PiperOrigin-RevId: 259786320
Change-Id: I71fd5b68be9c10bf47f576c3020af17f80212178
diff --git a/spdy/core/priority_write_scheduler.h b/spdy/core/priority_write_scheduler.h
index bcacb25..e438583 100644
--- a/spdy/core/priority_write_scheduler.h
+++ b/spdy/core/priority_write_scheduler.h
@@ -258,6 +258,8 @@
   // Returns the number of ready streams.
   size_t NumReadyStreams() const override { return num_ready_streams_; }
 
+  size_t NumRegisteredStreams() const override { return stream_infos_.size(); }
+
   SpdyString DebugString() const override {
     return SpdyStrCat(
         "PriorityWriteScheduler {num_streams=", stream_infos_.size(),