Fix a crash in HTMLFormElement::prepareForSubmission.

BUG=297478
TEST=automated with ASAN.

Review URL: https://chromiumcodereview.appspot.com/24910003

git-svn-id: svn://svn.chromium.org/blink/trunk@158428 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Node::InsertionNotificationRequest HTMLFormElement::insertedInto(ContainerNode* insertionPoint)
{
    HTMLElement::insertedInto(insertionPoint);
    if (insertionPoint->inDocument())
        this->document().didAssociateFormControl(this);
    return InsertionDone;
}

Node::InsertionNotificationRequest HTMLFormElement::insertedInto(ContainerNode* insertionPoint)
{
    HTMLElement::insertedInto(insertionPoint);
    if (insertionPoint->inDocument())
        this->document().didAssociateFormControl(this);
    return InsertionDone;
}
