Don't always full invalidation on 'position' style change

Move position changed case from
RenderStyle::diffNeedsFullLayoutAndPaintInvalidation()
to RenderStyle::diffNeedsFullLayout().

Our layout code can issue invalidations if actually needed.

BUG=408697
TEST=fast/repaint/position-change-keeping-geometry.html

Review URL: https://codereview.chromium.org/536183002

git-svn-id: svn://svn.chromium.org/blink/trunk@181423 bbb929c8-8fbe-4397-9dbb-9b2b20218538