Revert "Don't assume we have a worker_thread_ on linux (for now)"
Revert "Don't assume we have a worker_thread_ on linux (for now)"

This reverts commit 9ba33f1ce99343cf2a704324598aa9817b2c30ac.

Reason for revert: Breaks compilation on linux

Original change's description:
> Don't assume we have a worker_thread_ on linux (for now)
> 
> Tbr: mbonadei@webrtc.org
> No-Try: true
> Bug: none
> Change-Id: I0dca1e54b610b63651235a83ec80f0e7d76f51c4
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/173085
> Reviewed-by: Tommi <tommi@webrtc.org>
> Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
> Commit-Queue: Tommi <tommi@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#31019}

TBR=mbonadei@webrtc.org,tommi@webrtc.org

Change-Id: I860e98187364fdc69faf373d67e39e6bcfb1d4e9
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: none
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/173089
Reviewed-by: Artem Titov <1e27363ccc366a7ff0fdde48204d68d5ebfeccc5@webrtc.org>
Commit-Queue: Artem Titov <1e27363ccc366a7ff0fdde48204d68d5ebfeccc5@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#31020}

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