FormAssociatedElement shouldn't create out-of-tree FormAttributeTargetObserver.

This change addes an inDocument() check to prevent the observer creation
for out-of-document elements. It also adds an ASSERT() to clarify the invariant.

BUG=261609
TEST=fieldset-shadowdom-crash.html
R=tkent,inferno

Review URL: https://chromiumcodereview.appspot.com/19699011

git-svn-id: svn://svn.chromium.org/blink/trunk@154723 bbb929c8-8fbe-4397-9dbb-9b2b20218538