CSS font-display: expose display property on FontFace

This adds "display" attribute to FontFace interface that reflects the
font-display value of corresponding @font-face CSS rule. Also, this adds
"display" field to FontFaceDescriptors dictionary, to make it possible
to construct a FontFace with specified font-display value. These
are still protected behind the experimental features flag.

This matches Firefox's experimental implementation.

Spec discussion: https://github.com/tabatkins/specs/issues/54


Review-Url: https://codereview.chromium.org/2640753002
Cr-Commit-Position: refs/heads/master@{#444617}
7 files changed