🎈 Inform the client when a Custom Tab has been resized.

Change-Id: I00a9617d8c032b5bb5b80530e00a53c77f33f8b8
Bug: 1210827
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3109966


Auto-Submit: Peter Conn <peconn@chromium.org>
Commit-Queue: Shimi Zhang <ctzsm@chromium.org>
Reviewed-by: Shimi Zhang <ctzsm@chromium.org>
Cr-Commit-Position: refs/heads/main@{#914753}