Add Lighthouse team to list of people cc-ed on metrics changelog.

Change-Id: Ie9559c780d2b87365bb2b69c2111507e2da87bf9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2134503Reviewed-by: Paul Irish <paulirish@chromium.org>
Commit-Queue: Connor Clark <cjamcl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756309}

WATCHLISTS

@@ -2696,6 +2696,7 @@
     'source_idls': ['jmedley+watch@chromium.org'],
     'speed_metrics_changelog': ['igrigorik@chromium.org',
                                 'kayce@chromium.org',
+                                'lighthouse-eng+watch-speed-metrics@google.com',
                                 'rviscomi@chromium.org',
                                 'sullivan@chromium.org'],
     'spellcheck': ['rlp+watch@chromium.org',