Fix fetch-canvas-tainting.html to not mix up origins and transferables.

The signature of Window's postMessage is message, target origin,
transferables; not message, transferables, target origin.

https://html.spec.whatwg.org/multipage/comms.html#dom-window-postmessage

BUG=

Review URL: https://codereview.chromium.org/651603004

git-svn-id: svn://svn.chromium.org/blink/trunk@184230 bbb929c8-8fbe-4397-9dbb-9b2b20218538