Set up `style` mutation monitoring

Fixes #2008
This works by storing the style and its copy onto the component, and do the appropriate comparison during `_updateDOMProperties`.
This doesn't work if between two style settings, we set the style to some other random objects. But I think this'll do.

Also, rather than storing styleMutationWarning[ownerName], I guess I could have hashed ownerName + stringified style1 and style2.
(any reason for two underscores instead of one?)