Allow setting a delay for a response.

This serves as infrastructure for testing that timing metrics
measure what they're supposed to
(see https://chromium-review.googlesource.com/c/chromium/src/+/3815441)

PiperOrigin-RevId: 468108169
diff --git a/quiche/quic/tools/quic_memory_cache_backend.h b/quiche/quic/tools/quic_memory_cache_backend.h
index 7dfd372..ccdaf59 100644
--- a/quiche/quic/tools/quic_memory_cache_backend.h
+++ b/quiche/quic/tools/quic_memory_cache_backend.h
@@ -121,6 +121,12 @@
       spdy::Http2HeaderBlock response_headers, absl::string_view response_body,
       QuicBackendResponse::SpecialResponseType response_type);
 
+  // Finds a response with the given host and path, and assign it a simulated
+  // delay. Returns true if the requisite response was found and the delay was
+  // set.
+  bool SetResponseDelay(absl::string_view host, absl::string_view path,
+                        QuicTime::Delta delay);
+
   // Sets a default response in case of cache misses.  Takes ownership of
   // 'response'.
   void AddDefaultResponse(QuicBackendResponse* response);