Call ContainerNode::checkForSiblingStyleChanges() for Element insertion / removal only

Call ContainerNode::checkForSiblingStyleChanges() for Element insertion /
removal only. Other kind of changes (e.g. Text insertion / removal / change)
cannot cause sibling style invalidation.

This is inspired by WebKit r154957 from antti@apple.com:
http://trac.webkit.org/changeset/154957

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/395633007

git-svn-id: svn://svn.chromium.org/blink/trunk@179405 bbb929c8-8fbe-4397-9dbb-9b2b20218538