Fix debug assert for empty document loads during load deferral.

The defersLoading() assertion in DocumentLoader::finishedLoading() doesn't take
into account that we don't have a way to block empty loads. This patch updates
the assertion to better match the other defersLoading() assertions in 
DocumentLoader.

BUG=331627

Review URL: https://codereview.chromium.org/125103005

git-svn-id: svn://svn.chromium.org/blink/trunk@164526 bbb929c8-8fbe-4397-9dbb-9b2b20218538