DevTools: CSS.getPlatformFontsForNode() return cssFamilyName as well

This patch enhances CSS.getPlatformFontsForNode method from protocol to
return a matched css family name as well.

BUG=135489

Review URL: https://chromiumcodereview.appspot.com/22903030

git-svn-id: svn://svn.chromium.org/blink/trunk@156633 bbb929c8-8fbe-4397-9dbb-9b2b20218538