SVGs and child nodes referenced from local SVG sprite count towards "Avoid an excessive DOM size"

#### Provide the steps to reproduce
1. Run LH on https://andro4all.com/

#### What is the current behavior?
SVGs pointing to a local svg are adding nodes like they were the whole svg.

```html

```

becomes

```html

```

So, lighthouse evaluates this nodes too.

#### What is the expected behavior?
This svgs are pointing to a shared sprite SVG, so, lighthouse doesn't have to add the nodes inside the use tag. This rule penalizes reusing svgs instead of adding a new svg all the time.

Replacing this SVGs for dummy nodes in this page, for instance, like

```html



```

decreases the total by 376 nodes.

#### Environment Information
* Affected Channels: Devtools
* Lighthouse version: Lighthouse 4.0.0-beta
* Node.js version: macOS