Auto merge of #23340 - asajeffrey:gc-threshold-parameters-are-percentages, r=jdm

Express GC thresholds as percentages

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According to https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey/JSAPI_reference/JS_GetGCParameter, the GC thresholds are percentages.

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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 #23076
- [X] These changes do not require tests because we don't test prefs.json

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resources/prefs.json

@@ -42,8 +42,8 @@
   "js.ion.enabled": true,
   "js.ion.offthread_compilation.enabled": true,
   "js.ion.unsafe_eager_compilation.enabled": false,
-  "js.mem.gc.allocation_threshold_avoid_interrupt_factor": 1,
-  "js.mem.gc.allocation_threshold_factor": 1,
+  "js.mem.gc.allocation_threshold_avoid_interrupt_factor": 100,
+  "js.mem.gc.allocation_threshold_factor": 100,
   "js.mem.gc.allocation_threshold_mb": 30,
   "js.mem.gc.compacting.enabled": true,
   "js.mem.gc.decommit_threshold_mb": 32,