Add W3C standard Custom Elements to /tools/domprops.json

[Danny-Engelman]

enhancement

Add standard W3C Custom Elements to /tools/domprops.json, so they do not get mangled

"customElements", "define", "observedAttributes", "attributeChangedCallback", "connectedCallback", "isConnected", "disconnectedCallback", "adoptedCallback"

https://html.spec.whatwg.org/multipage/custom-elements.html#custom-elements

[alexlamsl]

This is similar to #2343 in that someone needs to put together a Pull Request after running tools/props.html on a list of web browsers to update the list of keywords to avoid.

[Danny-Engelman]

props.html does not pick up most of these (Chrome 65)

So I presume they will be erased again when someone PRs props.html output

I have added them to my uglify-config.json to be sure:

  "mangle": {
    "reserved": [
      "customElements",
      "define",
      "observedAttributes",
      "attributeChangedCallback",
      "connectedCallback",
      "isConnected",
      "disconnectedCallback",
      "adoptedCallback"
    ],