OS(MACOSX) and USE(WEBAUDIO_FFMPEG) are mutually exclusive.

Simplify platform/audio to remove unnecessary tests.

Review URL: https://codereview.chromium.org/110013008

git-svn-id: svn://svn.chromium.org/blink/trunk@164605 bbb929c8-8fbe-4397-9dbb-9b2b20218538