Speculative fix for ImageDocument crash.

HTMLImageElement::cachedImage can be null.
This CL has no tests. Unable to reproduce the crash of crbug.com/379720
locally.

BUG=379720

Review URL: https://codereview.chromium.org/314633002

git-svn-id: svn://svn.chromium.org/blink/trunk@175375 bbb929c8-8fbe-4397-9dbb-9b2b20218538