Ref transport and stream before init of chttp2_stream structure.

The stream structure is really big and by the time we get to
ref the stream and the transport, the fields are out of cache.
We need to ref them immediately, before starting to initialize
the stream.

This commit introduces a 0-byte Reffer so that we can
ref the streams on time before the field initializers.

src/core/ext/transport/chttp2/transport/chttp2_transport.cc

@@ -645,17 +645,22 @@ void grpc_chttp2_stream_unref(grpc_chttp2_stream* s) {
 }
 #endif
 
-grpc_chttp2_stream::grpc_chttp2_stream(grpc_chttp2_transport* t,
+grpc_chttp2_stream::Reffer::Reffer(grpc_chttp2_stream* s) {
-                                       grpc_stream_refcount* refcount,
-                                       const void* server_data,
-                                       gpr_arena* arena)
-    : t(t), refcount(refcount), metadata_buffer{{arena}, {arena}} {
   /* We reserve one 'active stream' that's dropped when the stream is
      read-closed. The others are for Chttp2IncomingByteStreams that are
      actively reading */
-  GRPC_CHTTP2_STREAM_REF(this, "chttp2");
+  GRPC_CHTTP2_STREAM_REF(s, "chttp2");
-  GRPC_CHTTP2_REF_TRANSPORT(t, "stream");
+  GRPC_CHTTP2_REF_TRANSPORT(s->t, "stream");
+}
 
+grpc_chttp2_stream::grpc_chttp2_stream(grpc_chttp2_transport* t,
+                                       grpc_stream_refcount* refcount,
+                                       const void* server_data,
+                                       gpr_arena* arena)
+    : t(t),
+      refcount(refcount),
+      reffer(this),
+      metadata_buffer{{arena}, {arena}} {
   if (server_data) {
     id = static_cast<uint32_t>((uintptr_t)server_data);
     *t->accepting_stream = this;

src/core/ext/transport/chttp2/transport/internal.h

@@ -510,6 +510,12 @@ struct grpc_chttp2_stream {
   void* context;
   grpc_chttp2_transport* t;
   grpc_stream_refcount* refcount;
+  // Reffer is a 0-len structure, simply reffing `t` and `refcount` in its ctor
+  // before initializing the rest of the stream, to avoid cache misses. This
+  // field MUST be right after `t` and `refcount`.
+  struct Reffer {
+    explicit Reffer(grpc_chttp2_stream* s);
+  } reffer;
 
   grpc_closure destroy_stream;
   grpc_closure* destroy_stream_arg;