Allow doubles for ProgressEvent's loaded and total

This allows specifications to use ProgressEvents while not necessarily giving away the exact number of bytes, while maintaining a granularity greater than just 1 part in 100. See discussion in webmachinelearning/writing-assistance-apis#15 for the concrete use case.

Author: domenic

xhr.bs

@@ -1753,14 +1753,14 @@ interface ProgressEvent : Event {
   constructor(DOMString type, optional ProgressEventInit eventInitDict = {});
 
   readonly attribute boolean lengthComputable;
-  readonly attribute unsigned long long loaded;
-  readonly attribute unsigned long long total;
+  readonly attribute double loaded;
+  readonly attribute double total;
 };
 
 dictionary ProgressEventInit : EventInit {
   boolean lengthComputable = false;
-  unsigned long long loaded = 0;
-  unsigned long long total = 0;
+  double loaded = 0;
+  double total = 0;
 };
 </pre>