Refactoring: Move loading-fallback stuff to CSSCustomFontData

CustomFontData for loading fallback font is always CSSCustomFontData,
so it's better to move those logic to CSSCustomFontData.

TEST=none, just refactoring
BUG=none

Review URL: https://codereview.chromium.org/184043035

git-svn-id: svn://svn.chromium.org/blink/trunk@168710 bbb929c8-8fbe-4397-9dbb-9b2b20218538