FontFace should return the same Promise object over multiple calls

This makes FontFace#loaded and #load() return the same instance of
promise on every call. By using ScriptPromiseProperty, we can avoid
a circular reference.

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

Review URL: https://codereview.chromium.org/440313002

git-svn-id: svn://svn.chromium.org/blink/trunk@179801 bbb929c8-8fbe-4397-9dbb-9b2b20218538