Worker: Remove a runtime flag for nested dedicated workers

Nested dedicated workers has been enabled by default since Chrome 69:
https://www.chromestatus.com/feature/6080438103703552

Bug: 31666
Change-Id: I12785dd8c1d5582ae0d069b5b6993ad50c6be9b3
Reviewed-on: https://chromium-review.googlesource.com/c/1325596
Reviewed-by: Nate Chapin <japhet@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608244}
7 files changed