Add "loaded" attribute of FontFace

This attribute is a Promise which is fulfilled or rejected when the
FontFace become "loaded" or "error" state. This provides a way to
observe font loads without calling load().

Since we cannot hold a ScriptPromise as a member of FontFace (that will
cause memory leak), the attribute returns new Promise every time.

spec: http://dev.w3.org/csswg/css-font-loading/#dom-fontface-loaded

TEST=fast/css/fontface-methods.html
BUG=

Review URL: https://codereview.chromium.org/286903008

git-svn-id: svn://svn.chromium.org/blink/trunk@174832 bbb929c8-8fbe-4397-9dbb-9b2b20218538