Support Custom Elements v1 in attachShadow

After renaming CustomElement to CustomElementV0, it looks the support of Custom
Elements v1 in attachShadow has been forgotten. Fix it.

BUG=531990,621942

Review-Url: https://codereview.chromium.org/2092703002
Cr-Commit-Position: refs/heads/master@{#401779}
2 files changed