Revert 169394 "Node::setTextContent should avoid work when nothi..."

Might have broken countless layout tests (ok I am exaggerating) on almost all platforms

> Node::setTextContent should avoid work when nothing changes
> 
> We already had code to handle this case, which is shared
> by editing as well as HTMLElement::setInnerText.
> 
> replaceChildrenWithText does the right thing to avoid
> doing work when the text content hasn't changed, etc.
> 
> BUG=352836
> R=jchaffraix@chromium.org
> 
> Review URL: https://codereview.chromium.org/200763006

TBR=eseidel@chromium.org

Review URL: https://codereview.chromium.org/202883002

git-svn-id: svn://svn.chromium.org/blink/trunk@169401 bbb929c8-8fbe-4397-9dbb-9b2b20218538