Rubber-stamped by Eric Seidel.

        Fix up some compositing layout tests.

        * compositing/layers-inside-overflow-scroll.html:
        * compositing/self-painting-layers.html:
        Use the 'loadeddata' event, instead of 'load', to know when the video loads.

        * platform/mac/compositing/repaint/content-into-overflow-expected.checksum:
        * platform/mac/compositing/repaint/content-into-overflow-expected.png:
        * platform/mac/compositing/repaint/overflow-into-content-expected.checksum:
        * platform/mac/compositing/repaint/overflow-into-content-expected.png:
        * platform/mac/compositing/self-painting-layers-expected.checksum:
        * platform/mac/compositing/self-painting-layers-expected.png:
        Update with correct pixel results.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@51478 268f45cc-cd09-0410-ab3c-d52691b4dbfc