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

Add start_time to resource timing.

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`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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components/script/dom/performanceresourcetiming.rs

@@ -55,7 +55,6 @@ pub struct PerformanceResourceTiming {
     decoded_body_size: u64, //size in octets
 }
 
-// TODO(#21254): startTime
 // TODO(#21255): duration
 // TODO(#21269): next_hop
 // TODO(#21264): worker_start
@@ -115,7 +114,7 @@ impl PerformanceResourceTiming {
             entry: PerformanceEntry::new_inherited(
                 DOMString::from(url.into_string()),
                 DOMString::from("resource"),
-                0.,
+                resource_timing.start_time as f64,
                 0.,
             ),
             initiator_type: initiator_type,