Merge updateOutOfFlowPositioned early out into RenderLayer::styleChanged

Instead of hiding the early out check for when the position hasn't changed
inside updateOutOfFlowPositioned move it into styleChanged and add an
ASSERT that the diff is >= StyleDifferenceLayout.

This and following patches aim to make it more clear how much code we
really run for each change type inside RenderLayer::styleChanged.

BUG=335703

Review URL: https://codereview.chromium.org/146693012

git-svn-id: svn://svn.chromium.org/blink/trunk@166073 bbb929c8-8fbe-4397-9dbb-9b2b20218538