Rename FontLoader to FontFaceSet and Document.fontloader to Document.fonts.

This is the first step to support the new Promise-based draft spec.
http://dev.w3.org/csswg/css-font-load-events/

BUG=53213

Review URL: https://chromiumcodereview.appspot.com/23620044

git-svn-id: svn://svn.chromium.org/blink/trunk@157676 bbb929c8-8fbe-4397-9dbb-9b2b20218538