[roll] Roll fuchsia [netstack3] Add Serializer::serialize_new_buf

The new method allows to serialize a packet into a new buffer without
destroying the serializer. It will be used to implement multicast
packet loopback, which requires serializing the packet twice.

Original-Bug: 42058186
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1056075
Original-Revision: 50acbeaf8930eb7e18b081b289a97bc7463dd95e
GitOrigin-RevId: 0088c033836e43ce61c19a656cec565383573d4c
Change-Id: Ib162278b2d3f17c1ec7941a474afe4dba5f2785d
diff --git a/stem b/stem
index 9332c5f..ffced55 100644
--- a/stem
+++ b/stem
@@ -11,7 +11,7 @@
              githooks="integration/git-hooks"
              remote="https://fuchsia.googlesource.com/fuchsia"
              gerrithost="https://fuchsia-review.googlesource.com"
-             revision="75bbb9ab24718b8dc233f68c780a019d432cf663"/>
+             revision="50acbeaf8930eb7e18b081b289a97bc7463dd95e"/>
   </projects>
   <hooks>
     <hook name="install-environment"