Adds an OnFrameHeader() callback to Http2VisitorInterface.

PiperOrigin-RevId: 371390069
Change-Id: Icb2bb06ac19b42077e61c45c8d89ae7255030ee3
diff --git a/http2/adapter/http2_visitor_interface.h b/http2/adapter/http2_visitor_interface.h
index a14f434..12729e7 100644
--- a/http2/adapter/http2_visitor_interface.h
+++ b/http2/adapter/http2_visitor_interface.h
@@ -53,6 +53,12 @@
   // Called when a connection-level processing error has been encountered.
   virtual void OnConnectionError() = 0;
 
+  // Called when the header for a frame is received.
+  virtual void OnFrameHeader(Http2StreamId stream_id,
+                             size_t length,
+                             uint8_t type,
+                             uint8_t flags) {}
+
   // Called when a non-ack SETTINGS frame is received.
   virtual void OnSettingsStart() = 0;