Don't use video to trigger compositing for layout tests
https://bugs.webkit.org/show_bug.cgi?id=113228

Reviewed by Adrienne Walker.

These tests used <video> to trigger compositing because a 3d identity
transform would have affected the overlap map differently. This is
no longer the case, so use translateZ(0) to trigger compositing instead,
which makes these tests more reliable and faster.

* compositing/geometry/limit-layer-bounds-clipping-ancestor.html:
* compositing/geometry/limit-layer-bounds-fixed-positioned.html:
* compositing/geometry/limit-layer-bounds-opacity-transition-expected.txt:
* compositing/geometry/limit-layer-bounds-opacity-transition.html:
* compositing/geometry/limit-layer-bounds-overflow-repaint.html:
* compositing/geometry/limit-layer-bounds-overflow-root.html:
* compositing/geometry/limit-layer-bounds-positioned-transition.html:
* compositing/geometry/limit-layer-bounds-positioned.html:
* compositing/geometry/limit-layer-bounds-transformed-overflow.html:
* compositing/geometry/limit-layer-bounds-transformed.html:
* compositing/layer-creation/scroll-partial-update-expected.txt:
* compositing/layer-creation/scroll-partial-update.html:
* platform/chromium-win/compositing/geometry/limit-layer-bounds-overflow-repaint-expected.txt:
* platform/chromium-win/compositing/layer-creation/scroll-partial-update-expected.txt:
* platform/chromium/TestExpectations:
* platform/chromium/compositing/layer-creation/scroll-partial-update-expected.txt:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@146814 268f45cc-cd09-0410-ab3c-d52691b4dbfc