Clear FormAssociatedElement::m_formAttributeTargetObserver on didMoveToNewDocument()

Before this change, didMoveToNewDocument() created new observer instance in didMoveToNewDocument().
This means that the element could have an observer even though it is out-of-document.
This doesn't matter in many cases like appendChild() since the element enters a document eventually.
But document.adoptNode() isn't in the case.

This change just clears the observer in didMoveToNewDocument() instead of refreshing it.
This works because the observer is created in insertedInto() if needed.

TEST=fieldset-adoptnode-crash.html
BUG=260087
R=tkent@chromium.org

Review URL: https://chromiumcodereview.appspot.com/19289008

git-svn-id: svn://svn.chromium.org/blink/trunk@154352 bbb929c8-8fbe-4397-9dbb-9b2b20218538