Remove unnecessary setNeedsCompositingUpdate(CompositingUpdateRebuildTree)

The RenderLayerCompositor can determine whether it needs to rebuild the tree.
We don't need to dirty this much state.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/450403003

git-svn-id: svn://svn.chromium.org/blink/trunk@180008 bbb929c8-8fbe-4397-9dbb-9b2b20218538