This looks like a good warning: [3523/5556] Building CXX object Source...iastream/MediaStreamTrackPrivate.cpp.o ../../Source/WebCore/platform/mediastream/MediaStreamTrackPrivate.cpp: In copy constructor ‘WebCore::MediaStreamTrackPrivate::MediaStreamTrackPrivate(const WebCore::MediaStreamTrackPrivate&)’: ../../Source/WebCore/platform/mediastream/MediaStreamTrackPrivate.cpp:50:1: warning: base class ‘class WTF::RefCounted<WebCore::MediaStreamTrackPrivate>’ should be explicitly initialized in the copy constructor [-Wextra] MediaStreamTrackPrivate::MediaStreamTrackPrivate(const MediaStreamTrackPrivate& other) ^~~~~~~~~~~~~~~~~~~~~~~
(not MSE-related)
*** This bug has been marked as a duplicate of bug 164636 ***