Triage few w3c/web-platform-tests/custom-elements/

Point w3c/web-platform-tests/custom-elements/ failures to
appropriate bug.

Review URL: https://codereview.chromium.org/547383002

git-svn-id: svn://svn.chromium.org/blink/trunk@181712 bbb929c8-8fbe-4397-9dbb-9b2b20218538