Add helpers and use StringPiece in HttpDecoderTest.

This will come handy when changing a number of tests to append garbage after
|input| and verify that processing stops before reaching end of input.

gfe-relnote: n/a, test-only change in QUIC v99-only class.
PiperOrigin-RevId: 254954119
Change-Id: I22496641c6788e7200b3ffce7ccdf52f12448826
diff --git a/quic/core/http/http_decoder.h b/quic/core/http/http_decoder.h
index 1df4620..e8332d7 100644
--- a/quic/core/http/http_decoder.h
+++ b/quic/core/http/http_decoder.h
@@ -8,6 +8,7 @@
 #include "net/third_party/quiche/src/quic/core/http/http_frames.h"
 #include "net/third_party/quiche/src/quic/core/quic_error_codes.h"
 #include "net/third_party/quiche/src/quic/platform/api/quic_export.h"
+#include "net/third_party/quiche/src/quic/platform/api/quic_string_piece.h"
 
 namespace quic {