Add a ScriptForbiddenScope to ContainerNode::parserAppendChild

We shouldn't run script for most of this function call. We also shouldn't
dispatch events so I moved more code into the block with the
NoEventDispatchAssertion.

Review URL: https://codereview.chromium.org/303043004

git-svn-id: svn://svn.chromium.org/blink/trunk@175030 bbb929c8-8fbe-4397-9dbb-9b2b20218538