Currently Underline Position is not calculated from Font Metrics, by
default Blink makes 1-pixel gap between baseline and underline
position. This patch adds support for to obtain Underline position
from Font Metrics

Skia patch is in review process for Underline Thickness and Position, added comments to Skia related Blink files which needs to update once Skia patch is committed.

BUG=347510

Review URL: https://codereview.chromium.org/182923003

git-svn-id: svn://svn.chromium.org/blink/trunk@171050 bbb929c8-8fbe-4397-9dbb-9b2b20218538