Fixed `--iron-container-width`

[WillSquire]

--iron-container-width had been hardcoded by setting the var in :host:

:host {
    --iron-container-width: 90%;
}

.iron-container {
    width: var(--iron-container-width);
}

Instead, set var() to use a fallback parameter:

.iron-container {
    width: var(--iron-container-width, 90%);
}

See: https://www.polymer-project.org/1.0/docs/devguide/styling#custom-css-properties