It's best if STACK_ALLOCATED classes use Member. Currently NotShared<> uses UntracedMember because it is constructed on a non-oilpan thread in OfflineAudioDestinationNode. That thread calls getChannelData on an AudioBuffer, which returns a NotShared<DOMFloat32Array>.

It is OK in this case, because the AudioBuffer is being kept alive via a CrossThreadPersistent<> handle, but it would be better if NotShared<> was not constructed in that case, so we could switch it to using Member instead of UntracedMember.

See discussion here: https://codereview.chromium.org/2707243006#msg66