[worklets] Split up InProcessWorkerMessagingProxy into a base+worker class.

This pulls out all of the lifetime logic for the WorkerThread into the base class ThreadedMessagingProxyBase.

This will allow us to re-use all of the lifetime logic (I've created a followup demo patch which does this https://codereview.chromium.org/2312493002/)

BUG=567358

Review-Url: https://codereview.chromium.org/2310673002
Cr-Commit-Position: refs/heads/master@{#417161}
7 files changed