[compositorworker] Register compositor proxies with proxy client

Each compositor proxy registers itself with its proxy client when created.

This populated set is later used to update the backing mutable state for each
compositor proxy.

TEST=These machinery is tested in the follow-up CL https://codereview.chromium.org/2041193005/

BUG=430155

Review-Url: https://codereview.chromium.org/1900423004
Cr-Commit-Position: refs/heads/master@{#399502}
9 files changed