Make 'mode' required parameter for Element.attachShadow
As 'mode' is a required parameter in the spec, omission should
throw an exception.
BUG=
531990
TEST=imported/web-platform-tests/shadow-dom/Element-interface-attachShadow-expected.txt
Committed:
https://crrev.com/cc5607e9bd52a8720bf7c9c25eb2239da8f83a8d
Cr-Commit-Position: refs/heads/master@{#374628}