Unify selection rect snapping logic

Unify the pixel snapping logic for selection rects by moving the logic
into selectionRectForText and having selectionRectForSimpleText and
selectionRectForComplexText respectively return precise values.

R=fmalita@chromium.org

Review URL: https://codereview.chromium.org/648633003

git-svn-id: svn://svn.chromium.org/blink/trunk@183632 bbb929c8-8fbe-4397-9dbb-9b2b20218538