compositor-worker: Introduce WebCompositorMutableState

This change introduces a wrapper around compositer-owned state that may
be mutated by a compositor worker.

This depends on crrev.com/1405993008

BUG=430155
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1447893002

Cr-Commit-Position: refs/heads/master@{#368273}
16 files changed