Add a ScriptForbiddenScope to ContainerNode::insertBeforeCommon

Add one to complement the NoEventDispatchAssertion.

Review URL: https://codereview.chromium.org/308593002

git-svn-id: svn://svn.chromium.org/blink/trunk@175032 bbb929c8-8fbe-4397-9dbb-9b2b20218538