Support nested workers in devtools

Bug: 829119
Change-Id: Idbc1dd0294174b457989893d11e66237a04c3ea5
Reviewed-on: https://chromium-review.googlesource.com/1005367
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Nate Chapin <japhet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551465}
12 files changed