Thread-checkers for PeerConnectionFactory::worker_thread_

Add RTC_DCHECKs that would ensure that PeerConnectionFactory::X_w() functions, intended to run on the worker_thread_, do so.

BUG=None

Review-Url: https://codereview.webrtc.org/3007153006
Cr-Commit-Position: refs/heads/master@{#19720}
diff --git a/webrtc/pc/peerconnectionfactory.cc b/webrtc/pc/peerconnectionfactory.cc
index 6da4669..2ff95f7 100644
--- a/webrtc/pc/peerconnectionfactory.cc
+++ b/webrtc/pc/peerconnectionfactory.cc
@@ -333,12 +333,15 @@
 }
 
 std::unique_ptr<RtcEventLog> PeerConnectionFactory::CreateRtcEventLog_w() {
+  RTC_DCHECK_RUN_ON(worker_thread_);
   return event_log_factory_ ? event_log_factory_->CreateRtcEventLog()
                             : rtc::MakeUnique<RtcEventLogNullImpl>();
 }
 
 std::unique_ptr<Call> PeerConnectionFactory::CreateCall_w(
     RtcEventLog* event_log) {
+  RTC_DCHECK_RUN_ON(worker_thread_);
+
   const int kMinBandwidthBps = 30000;
   const int kStartBandwidthBps = 300000;
   const int kMaxBandwidthBps = 2000000;