Fix crash in document.register when prototype is not an object

In CustomElementConstructorBuilder::validateOptions, check
if prototype is an object before attempting to cast.

This fixes the following test on Win7 dbg:
fast/dom/custom/document-register-reentrant-null-constructor.html
Where we'd crash because prototype was undefined.

BUG=295605

Review URL: https://codereview.chromium.org/90763002

git-svn-id: svn://svn.chromium.org/blink/trunk@162774 bbb929c8-8fbe-4397-9dbb-9b2b20218538