Disable selection and editing of inert nodes.

This patch makes inert nodes behave like user-select: none to prevent
selection and editing on them. Since text nodes must be recognized as
inert, it also moves Element::isInert to Node::isInert.

BUG=252071

Review URL: https://chromiumcodereview.appspot.com/23065006

git-svn-id: svn://svn.chromium.org/blink/trunk@156591 bbb929c8-8fbe-4397-9dbb-9b2b20218538