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As per a discussion on www-style [1], FontFace#ready is removed from the
spec [2], and FontFace#load() is changed to return a Promise which
fulfills when the font is ready.

[1] http://lists.w3.org/Archives/Public/www-style/2014Mar/0069.html
[2] http://dev.w3.org/csswg/css-font-loading/

TEST=fast/css/fontface-methods.html
BUG=53213

Review URL: https://codereview.chromium.org/184633005

git-svn-id: svn://svn.chromium.org/blink/trunk@169507 bbb929c8-8fbe-4397-9dbb-9b2b20218538