Make FontResource DecodeError status when font data is empty

This patch fixes a bug that loadingerror event is not reported when
font data is empty. FontResource::ensure{Custom,SVG}FontData should
set DecodeError status if data is empty (i.e. m_data is null).

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

Review URL: https://codereview.chromium.org/98723002

git-svn-id: svn://svn.chromium.org/blink/trunk@163061 bbb929c8-8fbe-4397-9dbb-9b2b20218538