[Font Load Events] Fix crash by calling FontFaceSet methods on unattached documents

Unattached documents do not have FontSelector. FontFaceSet cannot do
meaningful things for those, but at least it should not crash.
This patch adds check for Document::isActive() to each method entry.

BUG=53213
TEST=fast/css/fontfaceset-unattached-document.html

Review URL: https://codereview.chromium.org/141133008

git-svn-id: svn://svn.chromium.org/blink/trunk@166364 bbb929c8-8fbe-4397-9dbb-9b2b20218538