Remove comment about issue 368416

Since the issue was opened, the validity of removing
RenderObject::positionFromPaintInvalidationContainer
was questioned and it was determined that it's
probably not such a good idea anyway.

BUG=368416

Review URL: https://codereview.chromium.org/423943002

git-svn-id: svn://svn.chromium.org/blink/trunk@179078 bbb929c8-8fbe-4397-9dbb-9b2b20218538