Add font-stretch to FontDescription

Add FontStretch enum and field to FontDescription and FontTraitsMask.
This is the first step towards supporting the CSS font-stretch
property.

TEST=Source/platform/fonts/FontDescriptionTest.cpp
BUG=347765
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/183813005

git-svn-id: svn://svn.chromium.org/blink/trunk@168476 bbb929c8-8fbe-4397-9dbb-9b2b20218538