Revert 165846 "Build HTMLFormElement::m_associatedElements lazily"

> Build HTMLFormElement::m_associatedElements lazily
> 
> HTMLFormElement::associate() and disassociate() don't update
> m_associatedElements synchronously, and we build it by traversing a DOM tree on
> demand.
> 
> The code becomes simpler and we don't need complex functions like
> formElementIndexWithFormAttribute and formElementIndex any more.
> 
> BUG=334556
> TEST=none; no user-visible behavior changes.
> 
> Review URL: https://codereview.chromium.org/145353004

Suspected of causing issue 338764.

TBR=tkent@chromium.org

Review URL: https://codereview.chromium.org/148973002

git-svn-id: svn://svn.chromium.org/blink/trunk@165955 bbb929c8-8fbe-4397-9dbb-9b2b20218538