sizing attribute broken when using custom property apis

[dman777]

Description

The attribute sizing will be broken if --iron-image-width and --iron-image-height are used instead of width and height html attributes.

Note: this is with using placeholder="data:image/jpeg;base64,... base64 image

Expected outcome

Image will show with sizing="cover", --iron-image-width, and --iron-image-height

Actual outcome

Image does not show at all and image element takes no space on doc flow.

Live Demo

Steps to reproduce

Browsers Affected

• [X] Chrome
• [ ] Firefox
• [ ] Safari 9
• [ ] Safari 8
• [ ] Safari 7
• [ ] Edge
• [ ] IE 11
• [ ] IE 10