Update IWYU tool with a mapping file

Also apply IWYU to all .cc files in pc/, and correct BUILD file to match.
Note: Some files came out wrong when iwyu was applied. These are not included.

Bug: none
Change-Id: Ib5ea46b8fcc505414d0447cca7218ad3afc2e321
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/252280
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36064}
diff --git a/pc/rtp_transceiver_unittest.cc b/pc/rtp_transceiver_unittest.cc
index df6dd29..e5c7733 100644
--- a/pc/rtp_transceiver_unittest.cc
+++ b/pc/rtp_transceiver_unittest.cc
@@ -14,12 +14,15 @@
 
 #include <memory>
 
+#include "absl/strings/string_view.h"
 #include "absl/types/optional.h"
 #include "api/rtp_parameters.h"
 #include "media/base/fake_media_engine.h"
+#include "media/base/media_engine.h"
 #include "pc/test/mock_channel_interface.h"
 #include "pc/test/mock_rtp_receiver_internal.h"
 #include "pc/test/mock_rtp_sender_internal.h"
+#include "rtc_base/thread.h"
 #include "test/gmock.h"
 #include "test/gtest.h"