Adjust composited clip rects properly for sub-pixel

This one-line change properly accumulates sub-pixel into the clip rect for
composited layers. The adjustment isn't needed for the other branch in the
code as the paint rect has already been properly aligned.

BUG=343850

Review URL: https://codereview.chromium.org/173053002

git-svn-id: svn://svn.chromium.org/blink/trunk@167820 bbb929c8-8fbe-4397-9dbb-9b2b20218538