CSS loaded as incorrect charset unless explicitly declared

#### Provide the steps to reproduce
1. Run LH on https://shkspr.mobi/blog/
2. When the page reloads, the `✍` emoji incorrectly renders as `âœ`.

### Before LH

### After LH

#### What is the current behavior?

Why this occurs:

* My style.css contains `content: "✍";`
* It is served as `Content-Type: text/css;` with no charset.
* When the devtools reloads the page after Lighthouse runs, it corrupts the text - I think into Windows-1252
* If I add `@charset "UTF-8";` at the start of the CSS file, it renders correctly after Lighthouse runs.

#### What is the expected behavior?

[The CSS spec says](https://www.w3.org/TR/css-syntax-3/#input-byte-stream), if a text encoding can't be determined, assume UTF-8.

(I am unable to change the server to serve ` Content-Type: text/css; charset=utf-8`)

#### Environment Information
* Lighthouse version: Whatever ships with Chromium 78
* Chrome version: Version 78.0.3904.108 (Official Build) snap (64-bit)
* Operating System: Ubuntu

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