Add the CustomElementsRegistry interface behind the flag

This patch adds the CustomElementsRegistry interface defined in the
custom elements spec[1] behind the runtime flag.

The actual logic is not included in this patch.

[1] http://w3c.github.io/webcomponents/spec/custom/#custom-elements-api

BUG=594918

Review URL: https://codereview.chromium.org/1810973004

Cr-Commit-Position: refs/heads/master@{#385499}
13 files changed