Add RTC_PT_GUARDED_BY to sdp_handler_

As suggested in another review.
Also add one more guard, and some commentary.

Bug: None
Change-Id: I9b84453ff2533fe01d157fe84f07405d352e1dc7
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/235820
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35245}
diff --git a/pc/peer_connection.cc b/pc/peer_connection.cc
index eb153eb..0ca18aa 100644
--- a/pc/peer_connection.cc
+++ b/pc/peer_connection.cc
@@ -2880,6 +2880,7 @@
 }
 
 void PeerConnection::RequestUsagePatternReportForTesting() {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   message_handler_.RequestUsagePatternReport(
       [this]() {
         RTC_DCHECK_RUN_ON(signaling_thread());