Use AudioManager::CreateForTesting for unittests.
Use AudioManager::CreateForTesting for unittests.

This is a speculative fix for
AudioManagerTest.GetAssociatedOutputDeviceId where it is failing
to create core audio device enumerator presumably because COM is
not initialized on the audio thread.

BUG=604906
TBR=tommi,dalecurtis

Review URL: https://codereview.chromium.org/1897383003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#388379}

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