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

git-svn-id: svn://svn.chromium.org/blink/trunk@169394 bbb929c8-8fbe-4397-9dbb-9b2b20218538