Fix handling of Ligature when letter-spacing is present.

Blink is applying ligature feature when letter spacing is specified which is not correct as per the CSS
specification [1].

With this patch ligature is not applied when letter spacing is specified in CSS.

BUG=382381
TEST=fast/text/font-ligature-letter-spacing.html

1: http://dev.w3.org/csswg/css-text-3/#letter-spacing-property

Review URL: https://codereview.chromium.org/325673002

git-svn-id: svn://svn.chromium.org/blink/trunk@176539 bbb929c8-8fbe-4397-9dbb-9b2b20218538