Delete UpdateLayerPositionsFlags

This CL removes the last "flag" from update layer positions. I've also factored
the pagination update out into a separate walk. We should actually defer this
walk until we actually want to paint because paint is the primary (only?)
consumer of this information. There doesn't appear to be a reason to update it
eagerly after every layout.

R=esprehn@chromium.org
BUG=382548

Review URL: https://codereview.chromium.org/443773002

git-svn-id: svn://svn.chromium.org/blink/trunk@179657 bbb929c8-8fbe-4397-9dbb-9b2b20218538