Bug 1090821 - [Messages][Drafts] Can't save message draft with attachmen...

...t r=schung
Actually, the real fix is that it's non enumerable :)

If I don't do anything, when we call `this._renderer = ...` in `getAttachmentRenderer`, it gets "writable" and "enumerable" automatically. So here, I'm defining the property with "writable: true, enumerable: false". 

I mark it "writable" because I need to set it later on in _getAttachmentRenderer. "enumerable: false" is the default, that's why I did not add it here and added a comment instead. If you prefer, I can add it even if it's the default to make it explicit?

See http://www.w3.org/TR/html5/infrastructure.html#safe-passing-of-structured-data for the "structured clone" algorithm that is used in IndexedDB. What's important here is step 8:

> If deep clone is set to own, then, for each *enumerable* own property in input, run the following steps

Hope this is clearer!