[Font Load Events] Fix crash on FontFaceSet methods when document is destroyed

It is possible that when a method of FontFaceSet is called, its owner
document is already destroyed. We should check existence of document at
each method entry.

BUG=341330
TEST=fast/css/fontfaceset-unattached-document.html

Review URL: https://codereview.chromium.org/132753007

git-svn-id: svn://svn.chromium.org/blink/trunk@166674 bbb929c8-8fbe-4397-9dbb-9b2b20218538