Fix arithmetic mode when filter primitive subregion is applied.

BUG=337576

Review URL: https://codereview.chromium.org/146123003

git-svn-id: svn://svn.chromium.org/blink/trunk@165693 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Rename registerFormElement, removeFormElement, registerImgElement, and removeImgElement of HTMLFormElement.

These names are not good.
 - "register" and "remove"
   They should be register-unregister, or add-remove.
 - "FormElement" is confusing. It should be form-associated element, form
   control, or something.

This CL simplifies them:
 - registerFormElement -> associate
 - removeFormElement -> disassociate
 - registerImgElement -> associate
 - removeImgElement -> disassociate

We have a plan to stop updating element lists in register*Element and
remove*Element. So using "register" and "remove" will be inappropriate.

Also, this CL changes their pointer arguments to references because
these arguments never be null.


BUG=334556
TEST=none; no behavior changes.

Review URL: https://codereview.chromium.org/146103005

git-svn-id: svn://svn.chromium.org/blink/trunk@165694 bbb929c8-8fbe-4397-9dbb-9b2b20218538