Add intentional optimization comment in Node::setTextContent

SPEC: http://dom.spec.whatwg.org/#dom-node-textcontent

Current spec describes to use new text node
on setting Node.textContent
However, current implementation added early return
for optimization purpose.

So this CL added a note as well as crbug pointer
to describe those explicitly.

TEST=LayoutTests/fast/dom/Node/textContent.html

Review URL: https://codereview.chromium.org/383163003

git-svn-id: svn://svn.chromium.org/blink/trunk@178386 bbb929c8-8fbe-4397-9dbb-9b2b20218538