Rename 'formElement' in FormController.

Rename the following functions and a field:
 - FormController::registerFormElementWithState -> registerStatefulFormControl
 - FormController::unregisterFormElementWithState -> unregisterStatefulFormControl
 - FormController::m_formElementsWithState -> m_formControls

'formElement' is very confusing. It sounds like a <form> element, but it doesn't.

Also, this CL converts non-null pointer arguments to references.

BUG=none
TEST=none

Review URL: https://codereview.chromium.org/146533002

git-svn-id: svn://svn.chromium.org/blink/trunk@165821 bbb929c8-8fbe-4397-9dbb-9b2b20218538