Make DedicatedWorker classes safe for use off the main thread.

This is in preparation for nested dedicated workers.

Bug: 829119

Change-Id: Ifedf3a9b3d57a2e88b294280f148d25b6071a170
Reviewed-on: https://chromium-review.googlesource.com/993773
Commit-Queue: Nate Chapin <japhet@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548229}
5 files changed