Add MockSpdyHeadersHandler, mocking SpdyHeadersHandlerInterface.

This class will be used in cl/621527987 to provide expectations on serialized
headers by sending them through an Http2DecoderAdapter with a
MockSpdyFramerVisitorInterface that will return a MockSpdyHeadersHandler for
receiving headers.

PiperOrigin-RevId: 621613504
diff --git a/build/source_list.json b/build/source_list.json
index 373a3ea..c58090b 100644
--- a/build/source_list.json
+++ b/build/source_list.json
@@ -851,6 +851,7 @@
     "quiche/quic/test_tools/web_transport_resets_backend.h",
     "quiche/quic/test_tools/web_transport_test_tools.h",
     "quiche/spdy/test_tools/mock_spdy_framer_visitor.h",
+    "quiche/spdy/test_tools/mock_spdy_headers_handler.h",
     "quiche/spdy/test_tools/spdy_test_utils.h",
     "quiche/web_transport/test_tools/mock_web_transport.h"
   ],
@@ -945,6 +946,7 @@
     "quiche/quic/test_tools/test_ticket_crypter.cc",
     "quiche/quic/test_tools/web_transport_resets_backend.cc",
     "quiche/spdy/test_tools/mock_spdy_framer_visitor.cc",
+    "quiche/spdy/test_tools/mock_spdy_headers_handler.cc",
     "quiche/spdy/test_tools/spdy_test_utils.cc"
   ],
   "balsa_hdrs": [