Add a background-size:inherit to the placeholder

[herbalizer404]

Description

When you use the paper-card element which uses iron-image, you can't precise the "size" properties to the iron-image.
The only choice for you is to customize this css rule like that:
--paper-card-header-image {
background-size: cover;
}
BUT the iron-image background-size doesn't change because there is no attribute in the placeholder css rules like this one:
background-size: inherit;

Expected outcome

The iron-image placeholder of a paper-card image must inherit from this rule in the paper card definition:
--paper-card-header-image {
background-size: cover;
}

Actual outcome

The background-size: cover; doesn't work, cause there is no background-size: inherit; in the #placeholder css rules.

Steps to reproduce

1. Put a paper-card element in a page (eventually in an app-grid element), and add it an image, and a placeholder smaller than the image.
2. Open the page in a web browser.
3. What you'll see is that the placeholder shown is background-repeat instead of cover.