Remove argument from Element::setChildrenAffectBy* methods

No one should ever pass false to these methods, and indeed no one ever does,
and most of them don't take an argument already. This patch removes the
boolean argument and the if statement inside the methods which wasn't needed
since we always passed true (and would always go through the if statement).

Review URL: https://codereview.chromium.org/66213010

git-svn-id: svn://svn.chromium.org/blink/trunk@161744 bbb929c8-8fbe-4397-9dbb-9b2b20218538