bindings: Removes an unnecessary type conversion in V8DOMWrapper.

I don't see any reason to convert the internal pointer of type void* to Node* and convert it back to void*.

BUG=235436

Review URL: https://codereview.chromium.org/422233003

git-svn-id: svn://svn.chromium.org/blink/trunk@179151 bbb929c8-8fbe-4397-9dbb-9b2b20218538