Drag to desktop when using sizing="contain"

[Westbrook]

Description

The iron-image component uses CSS backgroundImage when containing the image or having the image cover it. While this is awesome for layout, it applies [hidden] the actual img tag making it inaccessible to dragging to the desktop.

Desired outcome

It is possible to drag the image to the desktop.

Actual outcome

You can't drag the image.

Live Demo

https://elements.polymer-project.org/elements/iron-image?view=demo:demo/index.html&active=iron-image

Steps to reproduce

1. Put a iron-image element in the page.
2. Set the sizing property to "contain".
3. Open the page in a web browser.

Conversation

Are there any accessibility issues with using something like position: relative; opacity: 0; width: 100%; height: 100%; in order to allow the image to be draggable but not effect the actual layout? In which case it would seem to make sense to either surface a CSS mixin, a component property to change the application of the [hidden], or possible both. I'd be happy to prepare a PR for what made the most sense to the health of the component.

[bicknellr]

See comment on #77.