Add mirroring of commits of master branch to main.

This is to prepare repositories for a migration of master branch to
main.

Bug: flutter/flutter#90476

.github/workflows/mirror.yml

@@ -0,0 +1,25 @@
+# Copyright 2013 The Flutter Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# Mirror master to main branches in the goldens repository.
+on:
+  push:
+    branches:
+      - 'master'
+
+jobs:
+  mirror_job:
+    permissions:
+      pull-requests: write
+    runs-on: ubuntu-latest
+    if: ${{ github.repository == 'flutter/goldens' }}
+    name: Mirror main branch to master branch
+    steps:
+      - name: Mirror action step
+        id: mirror
+        uses: google/mirror-branch-action@c6b07e441a7ffc5ae15860c1d0a8107a3a151db8
+        with:
+          github-token: ${{ secrets.FLUTTERGITHUBBOT_TOKEN }}
+          source: 'master'
+          dest: 'main'