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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---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [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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tests/wpt/metadata/cookies/schemeful-same-site/schemeful-navigation.tentative.html.ini

@@ -1,8 +1,3 @@
 [schemeful-navigation.tentative.html]
-  expected: TIMEOUT
   [Navigate cross-scheme]
-    expected: NOTRUN
-
-  [Navigate same-scheme]
-    expected: TIMEOUT
-
+    expected: FAIL

tests/wpt/metadata/cookies/schemeful-same-site/schemeful-subresource.tentative.html.ini

@@ -1,8 +1,3 @@
 [schemeful-subresource.tentative.html]
-  expected: TIMEOUT
   [Cross-scheme subresources cannot sent lax/strict cookies]
-    expected: NOTRUN
-
-  [Same-scheme subresources can send lax/strict cookies]
-    expected: TIMEOUT
-
+    expected: FAIL