Add a missing DCHECK to PlatformThread::SetPriority.

webrtc/base/platform_thread.cc

Index: webrtc/base/platform_thread.cc
diff --git a/webrtc/base/platform_thread.cc b/webrtc/base/platform_thread.cc
index b4724b99e951d486c2afa3003a7e2445b487cffe..d3a1a037fcdd4ee0d243ea471e409be1614dc8e6 100644
--- a/webrtc/base/platform_thread.cc
+++ b/webrtc/base/platform_thread.cc
@@ -279,6 +279,7 @@ bool PlatformThread::SetPriority(ThreadPriority priority) {
   if (run_function_) {
     // The non-deprecated way of how this function gets called, is that it must
     // be called on the worker thread itself.
+    RTC_DCHECK(!thread_checker_.CalledOnValidThread());
     RTC_DCHECK(spawned_thread_checker_.CalledOnValidThread());
   } else {
     // In the case of deprecated use of this method, it must be called on the