paper-menu: updated content while opened badly position it

I tried reproducing the first example of this page:

http://ember-basic-dropdown.com/docs/dropdown-events

Where a placeholder is displayed while the content is loaded.

In the case of `paper-menu` I have an issue where, once the content is loaded and rendered, the `calculatePosition` badly position it.

Turned out this is due to this changeset: https://github.com/miguelcobain/ember-paper/commit/d69255597e1c06834aebdc7ccad85d874bfdbb18

@miguelcobain Do you remember why you did this?

Start by reproducing the first example at http://ember-basic-dropdown.com/docs/dropdown-events with paper-menu and inspect calculatePosition when the placeholder is replaced by loaded content. Read changeset d69255597e1c06834aebdc7ccad85d874bfdbb18 and determine why it affects this case; done means the rendered menu is positioned correctly after its content updates.

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