mac: Make cast_sender build with the 10.11 SDK.
mac: Make cast_sender build with the 10.11 SDK.

With the 10.11 SDK, the compiler wants that no nullptrs are passed for
any of the callbacks in AudioFileInitializeWithCallbacks().  Add dummy
callbacks.  No intended behavior change.

BUG=517914

Review URL: https://codereview.chromium.org/1402603002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#353595}

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