Change focus ring rect calculation to use support subpixel text

We're currently (incorrectly) using pixelSnappedIntRect when
calculating focus ring rects. This wasn't a problem beore as text used 
integear advances however now that we support subpixel X-advances this
can cause focus rings to be a liitle too tight in some cases. Flooring
the left edge and ceiling the right guarantees that the content is fully
contained by the focus ring.

R=pdr@chromium.org
BUG=388426
TEST=fast/inline/continuation-outlines.html

Review URL: https://codereview.chromium.org/351563007

git-svn-id: svn://svn.chromium.org/blink/trunk@176889 bbb929c8-8fbe-4397-9dbb-9b2b20218538