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 <titovartem@webrtc.org>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31020}

video/receive_statistics_proxy.cc

diff --git a/video/receive_statistics_proxy.cc b/video/receive_statistics_proxy.cc
index 882bf71..223b943 100644
--- a/video/receive_statistics_proxy.cc
+++ b/video/receive_statistics_proxy.cc
@@ -769,7 +769,6 @@
 void ReceiveStatisticsProxy::RtcpPacketTypesCounterUpdated(
     uint32_t ssrc,
     const RtcpPacketTypeCounter& packet_counter) {
-#if !defined(WEBRTC_LINUX)
   if (!worker_thread_->IsCurrent()) {
     // RtpRtcp::Configuration has a single RtcpPacketTypeCounterObserver and
     // that same configuration may be used for both receiver and sender
@@ -792,7 +791,6 @@
   }
 
   RTC_DCHECK_RUN_ON(&main_thread_);
-#endif
   rtc::CritScope lock(&crit_);
   if (stats_.ssrc != ssrc)
     return;