Correctly accumulate sub-pixel offset into transform origin

In r164561 I added code to properly store and accumulate sub-pixel
offsets into composited layers to avoid pixel-snapping differences
between composited and non-composited content. I missed
accumulating this offset into the code to determine the center of
a composited renderer for use in transforms.

BUG=338674

Review URL: https://codereview.chromium.org/152453002

git-svn-id: svn://svn.chromium.org/blink/trunk@166371 bbb929c8-8fbe-4397-9dbb-9b2b20218538