Floor offset of focus ring rects in layered path of addChildFocusRingRects()

Floor the offset to match other addFocusRingRects() paths to
keep consistency, so that aligned children of focused elements
can result aligned focus ring rects.

BUG=405422
TEST=fast/sub-pixel/focus-ring-around-sub-pixel-layer.html
R=esprehn@chromium.org, jchaffraix@chromium.org, leviw@chromium.org

Review URL: https://codereview.chromium.org/488223002

git-svn-id: svn://svn.chromium.org/blink/trunk@180691 bbb929c8-8fbe-4397-9dbb-9b2b20218538