Change FontFaceSet#check() behavior when no matched font-face

When the font name passed to check() does not match any font face in the
FontFaceSet, current implementation always returns true.  As per the
latest spec draft [1], it should return true if it matches platform font
name, otherwise false.

[1] http://dev.w3.org/csswg/css-font-loading/#font-face-set-check

BUG=378989
TEST=fast/css/fontfaceset-check-platform-fonts.html

Review URL: https://codereview.chromium.org/324293003

git-svn-id: svn://svn.chromium.org/blink/trunk@176278 bbb929c8-8fbe-4397-9dbb-9b2b20218538