Implement nested dedicated workers

This change exposes the Worker() constructor to dedicated workers,
allowing them to create descendant workers.

Chrome feature entry:
https://www.chromestatus.com/feature/6080438103703552

Intent to Implement and Ship:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/KZx0i3e5nZM

Bug: 31666
Change-Id: I449978641f7ee4afba2a1087da91d84b78559abb
Reviewed-on: https://chromium-review.googlesource.com/953746
Reviewed-by: Ojan Vafai <ojan@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Nate Chapin <japhet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576225}
37 files changed