Revert of Oilpan: Make HTMLImageElement::m_form and HTMLFormElement::m_imageElements traceable. (https://codereview.chromium.org/294123014/)

Reason for revert:
Broke Oilpan build


Original issue's description:
> Oilpan: Make HTMLImageElement::m_form and HTMLFormElement::m_imageElements traceable.
> 
> This fixes flaky crash of fast/forms/form-submission-crash-successful-submit-button.html
> 
> BUG=357163
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=174658

TBR=keishi@chromium.org,oilpan-reviews@chromium.org,haraken@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=357163

Review URL: https://codereview.chromium.org/294233002

git-svn-id: svn://svn.chromium.org/blink/trunk@174659 bbb929c8-8fbe-4397-9dbb-9b2b20218538