Mac platform was not treating 8bit PCM audio as unsigned data.
Mac platform was not treating 8bit PCM audio as unsigned data.

If bits per channel is 8, open the stream as unsigned and use 128 as the silence value.

BUG=70730
TEST=media_unittests and manual testing of a bunch of audio files (8 bit and others).

Review URL: http://codereview.chromium.org/6689003

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@80181 0039d316-1c4b-4281-b951-d872f2087c98

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