Add COMPONENT to //ui/compositor

Useful for binary size attribution (amongst other things)

Change-Id: Id167b1c5d22811d48265b69b9f05c97b881ff75c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529122
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Ian Vollick <vollick@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642063}