Implement nested dedicated workers

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

Chrome feature entry:

Intent to Implement and Ship:!topic/blink-dev/KZx0i3e5nZM

Bug: 31666
Change-Id: I449978641f7ee4afba2a1087da91d84b78559abb
Reviewed-by: Ojan Vafai <>
Reviewed-by: Matt Falkenhagen <>
Reviewed-by: Kinuko Yasuda <>
Reviewed-by: Daniel Cheng <>
Reviewed-by: Hiroki Nakagawa <>
Commit-Queue: Nate Chapin <>
Cr-Commit-Position: refs/heads/master@{#576225}
37 files changed