Update platform/graphics tests to WebThread API

BUG=301514
R=hclam@chromium.org

Review URL: https://codereview.chromium.org/35223002

git-svn-id: svn://svn.chromium.org/blink/trunk@160262 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Convert the zOrder and normal flow lists to store stacking nodes.

This CL converts the zOrder and normal flow lists to store
RenderLayerStackingNodes instead of RenderLayer objects.

BUG=302120

Review URL: https://codereview.chromium.org/35263002

git-svn-id: svn://svn.chromium.org/blink/trunk@160263 bbb929c8-8fbe-4397-9dbb-9b2b20218538