[css-shadow-parts] Separate the mechanism to define part and theme

[rniwa]

We would like the mechanism to define ::part and ::theme be separated to let components decide which parts should be theme-able.

[rniwa]

Strawman proposal would be to add two content attributes: part and theme.

[tabatkins]

Yeah, I think I agree with this. I'd like to get a Polymer opinion, but I'm inclined to accept right now.

[daKmoR]

I'm in favor of this change :) would solve my use case as well :)