Move ThreadSafeDataTransport* to platform/

BUG=297477
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/27218006

git-svn-id: svn://svn.chromium.org/blink/trunk@159801 bbb929c8-8fbe-4397-9dbb-9b2b20218538