Add a missing DCHECK to PlatformThread::SetPriority.

This DCHECK is for the 'new and improved' way of setting thread priority.
What could happen is that code that's migrating over to the new method
might still have a lingering SetPriority call, that could incorrectly bind the
'spawned_thread_checker_' to the construction thread.

BUG=none

Review-Url: https://codereview.webrtc.org/2753423002
Cr-Commit-Position: refs/heads/master@{#17305}

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