Break dependency from core/platform/audio to modules/webaudio

This CL breaks the only dependency from core/platform/audio on
modules/webaudio. After this CL, we'll be able to move core/platform/audio into
blink_platform.dll.

R=kbr@chromium.org
BUG=297477

Review URL: https://codereview.chromium.org/25883005

git-svn-id: svn://svn.chromium.org/blink/trunk@159044 bbb929c8-8fbe-4397-9dbb-9b2b20218538