Auto merge of #24508 - pajamapants3000:21254, r=jdm

Add start_time to resource timing.

<!-- Please describe your changes on the following line: -->
`start_time` property added to `ResourceFetchTiming`, which enables the setting of `start_time` in the `PerformanceEntry` member of `PerformanceResourceTiming`.

Following the specification at https://w3c.github.io/resource-timing/#dfn-starttime, `start_time` is set to the value of `redirect_start` if redirection occurs and the timing allow check passes. Otherwise it has the same value as `fetch_start`.

---
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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 #21254

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- [x] There are tests for these changes OR
- [ ] These changes do not require tests because ___

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tests/wpt/mozilla/meta/MANIFEST.json

@@ -19634,7 +19634,7 @@
    "testharness"
   ],
   "mozilla/window_performance.html": [
-   "690870b7080e179481ca0255f7c30337e8b6636a",
+   "302073e8041763102d678326509d7ef0a1fb5c79",
    "testharness"
   ],
   "mozilla/window_performance_topLevelDomComplete.html": [

tests/wpt/mozilla/meta/mozilla/window_performance.html.ini

@@ -1,4 +0,0 @@
-[window_performance.html]
-  [window_performance]
-    expected: FAIL
-

tests/wpt/mozilla/tests/mozilla/window_performance.html

@@ -16,8 +16,13 @@ test(function() {
   assert_not_equals(entries.length, 0);
   assert_true(entries[0] instanceof PerformanceNavigationTiming);
 
-  // TODO(#21254) navigationTiming/startTime is not fully implemented yet, so this assertion will fail
-  assert_greater_than(entries[0].startTime, 0);
+  /* PerformanceResourceTiming */
+  var resourceEntries = window.performance.getEntriesByType("resource");
+  assert_not_equals(resourceEntries.length, 0);
+  assert_true(resourceEntries[0] instanceof PerformanceResourceTiming);
+  // no redirects => 0 < startTime == fetchStart
+  assert_greater_than(resourceEntries[0].startTime, 0);
+  assert_equals(resourceEntries[0].startTime, resourceEntries[0].fetchStart);
 
   var last = window.performance.now();
   assert_greater_than(last, 0);