WebAudioSourceProvider.h belongs in public/platform, not public/web

R=abarth@chromium.org, abarth
BUG=301947

Review URL: https://codereview.chromium.org/25362003

git-svn-id: svn://svn.chromium.org/blink/trunk@158553 bbb929c8-8fbe-4397-9dbb-9b2b20218538