Don't abort drags if drag image generation fails.

This unintuitively caused drags to end immediately if a page called
setDragImage with a detached node or one that hadn't been through layout
yet. There's nothing in the spec that says the drags should abort in
this case either, so this makes Blink more spec-conformant.

BUG=413795

Review URL: https://codereview.chromium.org/724663002

git-svn-id: svn://svn.chromium.org/blink/trunk@185343 bbb929c8-8fbe-4397-9dbb-9b2b20218538