Changing visibility shouldn't dirty stacking tree

Visibility doesn't figure into the algorithms in RenderLayerStackingNode.
There's no reason to dirty the stacking tree when visibility changes.

Review URL: https://codereview.chromium.org/309963002

git-svn-id: svn://svn.chromium.org/blink/trunk@175370 bbb929c8-8fbe-4397-9dbb-9b2b20218538