Move HRTFDatabaseLoader to WebThread API

BUG=301514
R=kbr@chromium.org,abarth@chromium.org

Review URL: https://codereview.chromium.org/57803003

git-svn-id: svn://svn.chromium.org/blink/trunk@161288 bbb929c8-8fbe-4397-9dbb-9b2b20218538