Allow attachShadow only for elements in the safelist

The spec is: http://w3c.github.io/webcomponents/spec/shadow/#methods-1
The spec discussion is: https://github.com/w3c/webcomponents/issues/110

BUG=531990

Review URL: https://codereview.chromium.org/1698183003

Cr-Commit-Position: refs/heads/master@{#375539}
4 files changed