HTMLMetaElement should not run script inside ::insertedInto

To eventually remove the ChildNodesLazySnapshot we need to stop allowing
script to run inside ::insertedInto. It's also important since even with
the lazy snapshot we still keep getting security bugs down there.

This starts the process of removal by switching HTMLMetaElement to use
a post insertion callback.

R=ojan@chromium.org,abarth@chromium.org
BUG=357435

Review URL: https://codereview.chromium.org/214003003

git-svn-id: svn://svn.chromium.org/blink/trunk@170267 bbb929c8-8fbe-4397-9dbb-9b2b20218538