Repaint as necessary when a composited layer has a positioned movement layout

r102952[1] introduced an optimization where repainting could be avoided when
a composited layer had a positioned movement layout. Unfortunately that patch
did not consider other invalidations that might require a repaint (eg. changing
the background).

This patch avoids the regression by only applying the optimization only when
the invalidation is layout related.

[1] http://trac.webkit.org/changeset/102952

BUG=326575,328749

Review URL: https://codereview.chromium.org/100383005

git-svn-id: svn://svn.chromium.org/blink/trunk@163926 bbb929c8-8fbe-4397-9dbb-9b2b20218538