Remove WebAudioSourceProvider.h from public/web.

This is a two side patch and the Chromium side has to be landed first.

BUG=None
TEST=None
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/26168002

git-svn-id: svn://svn.chromium.org/blink/trunk@159232 bbb929c8-8fbe-4397-9dbb-9b2b20218538