Fix two TSAN bugs exposed when running with -c opt:

net/tools/quic/quic_in_memory_cache.h

Index: net/tools/quic/quic_in_memory_cache.h
diff --git a/net/tools/quic/quic_in_memory_cache.h b/net/tools/quic/quic_in_memory_cache.h
index c42d15a34dbf5e9c7f13f09fa7c6d535e7da21d6..174a7d51635e268346b26506e0945f792223c65f 100644
--- a/net/tools/quic/quic_in_memory_cache.h
+++ b/net/tools/quic/quic_in_memory_cache.h
@@ -234,6 +234,10 @@ class QuicInMemoryCache {
   // A map from request URL to associated server push responses (if any).
   std::multimap<std::string, ServerPushInfo> server_push_resources_;
 
+  // Protects against concurrent access from test threads setting responses, and
+  // server threads accessing those responses.
+  mutable base::Lock response_mutex_;
+
   DISALLOW_COPY_AND_ASSIGN(QuicInMemoryCache);
 };