Add callbacks to ScriptCustomElementDefinition

This patch adds callbacks and observedAttributes to
ScriptCustomElementDefinition, as defined in the Element Definition
spec[1].

[1] https://html.spec.whatwg.org/multipage/scripting.html#element-definition

BUG=594918
TEST=imported/wpt/custom-elements/custom-elements-registry/define.html

Review-Url: https://codereview.chromium.org/2024073002
Cr-Commit-Position: refs/heads/master@{#398243}
9 files changed