Remove ChildNodesLazySnapshot and add ASSERTs to ChildNodeInsertionNotifier

Now that we no longer run script or dispatch unsafe events inside the
loop over ::insertedInto we no longer need the ChildNodesLazySnapshot
machinery and can add a NoEventDispatchAssertion and a ScriptForbiddenScope
around the insertedInto notification step.

This also makes the notify step always iterative.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/294253003

git-svn-id: svn://svn.chromium.org/blink/trunk@174708 bbb929c8-8fbe-4397-9dbb-9b2b20218538