Add mailmap entry for timothygu99@gmail.com

Change-Id: I3ec23a8969e311be3afe684df4a545bec0453449
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2971404Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Hannes Payer <hpayer@chromium.org>
Auto-Submit: Timothy Gu <timothygu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75348}

.mailmap

@@ -8,6 +8,7 @@
 #
 # See https://git-scm.com/docs/gitmailmap for more information.
 #
-# Please keep the list sorted.
+# Please keep the list sorted by the new email address.
 #
 Clemens Backes <clemensb@chromium.org> Clemens Hammacher <clemensh@chromium.org>
+Timothy Gu <timothygu@chromium.org> <timothygu99@gmail.com>