Auto merge of #29236 - servo:application-json, r=jdm

Allow displaying content with "application/json" mime type

For me this allows the WPT test
performance-timeline/tentative/include-frames-one-remote-child.sub.html
to match expected results. The wptserver is sending a 404 JSON response
because the URL that the test requests is not found.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #29136.
- [x] There are tests for these changes.

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