Refactoring: No need to handle font-family with multiple names in @font-face

Unlike font-family property, font-family descriptor in @font-face rule can
take only one family name. This patch removes the loop iterating over
the list of font-family value.
There is no behavior change, as CSSParser::createFontFaceRule() rejects
font-family descriptor with multiple names.

spec: http://www.w3.org/TR/css3-fonts/#font-family-desc

Test=Covered by fast/css/font-face-font-family-descriptor.html

Review URL: https://chromiumcodereview.appspot.com/21465003

git-svn-id: svn://svn.chromium.org/blink/trunk@155370 bbb929c8-8fbe-4397-9dbb-9b2b20218538