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This CL separates the work we do to update the GraphicsLayer geometry from the
work we do to rebuild the GraphicsLayer tree. We now update the geometry in
tree-order rather than paint order, which fixes a bug in how we're propagating
dirty bits during the tree walk. The bug fix is captured by the
reorder-z-with-style.html test added in this CL.

Review URL: https://codereview.chromium.org/215063006

git-svn-id: svn://svn.chromium.org/blink/trunk@170277 bbb929c8-8fbe-4397-9dbb-9b2b20218538