feat(icon): add support for viewbox size when registering an svg iconset

[alirezamirian]

angular material v1.x supports viewbox size and it's necessary for arbitrary icon sets.

[zeddever]

This requires changing the line:
https://github.com/angular/material2/blob/9062640b9a1ff98432218fc8ec59332ff154e781/src/lib/icon/icon-registry.ts#L466

to the code below:

const svgNode = this._setSvgAttributes(this._toSvgElement(iconElement));
if (iconElement.hasAttribute('viewBox') && !svgNode.hasAttribute('viewBox')) {
    svgNode.setAttribute('viewBox', iconElement.getAttribute('viewBox'));
}
return svgNode;

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