[Font Load Events] Partial implementation of SetClass operations for FontFaceSet

This adds has() and forEach() methods and size attribute to FontFaceSet.
These are indicated by SetClass(FontFace) extended attribute of the
FontFaceSet interface[1]. The set interface should match the ES6 Set
objects[2].

This patch also makes FontFaceSet accesses CSSSegmentedFontFaceCache
directly; now CSSSegmentedFontFaceCache is a common backend for
CSSFontSelector and FontFaceSet.

[1] http://dev.w3.org/csswg/css-font-load-events/#FontFaceSet-interface
[2] https://people.mozilla.org/~jorendorff/es6-draft.html#sec-set-objects

BUG=53213
TEST=fast/css/fontfaceset-set-operations.html

Review URL: https://codereview.chromium.org/137253005

git-svn-id: svn://svn.chromium.org/blink/trunk@165185 bbb929c8-8fbe-4397-9dbb-9b2b20218538