Auto merge of #15245 - andreastt:webdriver-caps, r=jdm

Correct capabilities returned by WebDriver server

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These patches aligns Servo’s capabilities with those in the [WebDriver standard](https://w3c.github.io/webdriver/webdriver-spec.html). See each commit’s message for further information.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because _Servo needs more work before it can pass WPT WebDriver tests_

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