Bug 1654112 - Update libwebrtc third party paths. r=ng

This removes media/signaling/src from ThirdPartyPaths.txt as that directory no
longer exists.

It also adds media/signaling/gtest/MockCall.h as the new SetAndGetRecordingState
from upstream libwebrtc causes a static-analysis error because RecordingState is
a non-param type.

Differential Revision: https://phabricator.services.mozilla.com/D108081

tools/rewriting/ThirdPartyPaths.txt

@@ -122,8 +122,7 @@ media/libyuv/
 media/mp4parse-rust/
 media/openmax_dl/
 media/openmax_il/
-media/webrtc/signaling/src/sdp/sipcc/
-media/webrtc/trunk/
+media/webrtc/signaling/gtest/MockCall.h
 mfbt/double-conversion/double-conversion/
 mfbt/lz4/.*
 mobile/android/geckoview/src/thirdparty/