Fix for input/datalist failure when datalist is created outside document

This patch fixes the failure, where datalist, when created in a shadowroot
outside the document, doesnot behave as expected. This patch changes
HTMLInputElement::resetListAttributeTargetObserver to  update the list
reference when it resets the observer.

TEST=fast/forms/datalist/datalist-inside-shadow-dom.html
BUG=391995

Review URL: https://codereview.chromium.org/371413004

git-svn-id: svn://svn.chromium.org/blink/trunk@178632 bbb929c8-8fbe-4397-9dbb-9b2b20218538