Specify a bit about how the initator field in requests should work

Author: jgraham

index.bs

@@ -3984,7 +3984,37 @@ The <code>Initiatior</code> type represents the source of a network request.
 <div algorithm>
 To <dfn>get the initiator</dfn> given |request|:
 
-1. TODO
+1. Let |type| be "<code>other</code>".
+
+1. If |request| is a [=CORS Preflight Request=], set |type| to
+   "<code>preflight</code>".
+
+1. TODO: Get the |type|. It's not quite clear how this ought to work; the CDP
+   data depends on whether the navigation was kicked off by the parser or by
+   script (so e.g. inserting an image from script causes the initiator to be
+   "<code>script</code>"), but that doesn't correspond to anything in Fetch.
+
+1. If |request|'s [=request/initiator-type=] is "<code>fetch</code>" or
+   "<code>xmlhttprequest</code>":
+
+  1. Let |stack trace| be the [=current stack trace=].
+
+  1. If |stack trace| has size of 1 or greater, let |line number| be value of the
+     <code>lineNumber</code> field in |stack trace|[0], and let |column number| be
+     the value of the <code>columnNumber</code> field |stack trace|[0]. Otherwise
+     let |line number| and |column number| be 0.
+
+  Otherwise, let |stack trace|, |column number|, and |line number| all be null.
+
+  TODO: Chrome includes the current parser position as column number / line
+  number for parser-inserted resources.
+
+1. Return a [=map=] matching the <code>NetworkInitiator</code> production, with
+   the |type| field set to "<code>type</code>", the <code>columnNumber</code>
+   field set to |column number| if it's not null, or omitted otherwise, the
+   <code>lineNumber</code> field set to |line number| if it's not null, or
+   omitted otherwise, the <code>stackTrace</code> field set to |stack trace| if
+   it's not null, or omitted otherwise.
 
 </div>