Add //third_party/mediapipe to CI watchlist

Bug: b/283522287
Change-Id: I5bed0c53b39cf00ad2c1b268dbf259b559946ad9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4654836
Auto-Submit: Robert Ogden <robertogden@chromium.org>
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Sophie Chang <sophiechang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1163733}

WATCHLISTS

@@ -722,6 +722,7 @@
                   '|third_party/fp16'\
                   '|third_party/gemmlowp'\
                   '|third_party/libzip'\
+                  '|third_party/mediapipe'\
                   '|third_party/neon_2_sse'\
                   '|third_party/ruy'\
                   '|third_party/tensorflow-text'\