Add SpdyFramerVisitorInterface::OnPriorityUpdate().

This method is not yet used.  It will be called from Http2FrameDecoder in the
next CL.

PiperOrigin-RevId: 348086749
Change-Id: Ie76e161b77fc16222461270a775d827d6e660c9a
diff --git a/quic/core/http/quic_headers_stream_test.cc b/quic/core/http/quic_headers_stream_test.cc
index dc6a46d..cd8120d 100644
--- a/quic/core/http/quic_headers_stream_test.cc
+++ b/quic/core/http/quic_headers_stream_test.cc
@@ -162,6 +162,11 @@
                int weight,
                bool exclusive),
               (override));
+  MOCK_METHOD(void,
+              OnPriorityUpdate,
+              (SpdyStreamId prioritized_stream_id,
+               absl::string_view priority_field_value),
+              (override));
   MOCK_METHOD(bool,
               OnUnknownFrame,
               (SpdyStreamId stream_id, uint8_t frame_type),