Implement :defined pseudo-class selector for Custom Elements

This patch implements :defined pseudo-class selector[1].

Adding use counter is in a separate patch, as it's updated more often
and needs other reviewers.

[1] https://html.spec.whatwg.org/multipage/scripting.html#selector-defined

BUG=594918

Review-Url: https://codereview.chromium.org/1989343002
Cr-Commit-Position: refs/heads/master@{#396139}
7 files changed