[Font Load Events] check() should return true if no matched font faces

When there are no downloadable faces to load (e.g. platform-only font),
everything is "loaded", so FontFaceSet#check() should return true.

spec: http://dev.w3.org/csswg/css-font-load-events/#font-face-set-check

BUG=53213
TEST=fast/css/fontfaceset-multiple-families.html

Review URL: https://codereview.chromium.org/155623002

git-svn-id: svn://svn.chromium.org/blink/trunk@166608 bbb929c8-8fbe-4397-9dbb-9b2b20218538