Add the HTTP parts

Author: martinthomson

api.bs

@@ -1686,7 +1686,7 @@ set on a response requesting that the user agent invoke the
 <a method for=Attribution>saveImpression()</a> API.
 
 <pre class=example id=ex-save-impression-header>
-Save-Impression: conversion-sites=("advertiser.example"), histogram-index=2, match-value=12, lifetime-days=7
+Save-Impression: conversion-sites=("advertiser.example"), conversion-callers=("intermediary.example"), histogram-index=2, match-value=12, lifetime-days=7
 </pre>
 
 The following keys are defined, corresponding to the members of
@@ -1702,6 +1702,14 @@ the {{AttributionImpressionOptions}} dictionary passed to
     [[RFC5890|Internationalized Domain Names]] therefore need to use [[RFC3492|punycode]].
     This key is optional. If not supplied, an empty set is saved for [=impression/Conversion Sites=].
   </dd>
+  <dt><dfn noexport><code>conversion-callers</code></dfn></dt>
+  <dd>
+    Value of <a dict-member for=AttributionImpressionOptions>conversionCallers</a>,
+    an [=structured header/inner list=] containing [=structured header/string|strings=].
+    Each string value includes a domain name using A-labels only;
+    [[RFC5890|Internationalized Domain Names]] therefore need to use [[RFC3492|punycode]].
+    This key is optional. If not supplied, an empty set is saved for [=impression/Conversion Callers=].
+  </dd>
   <dt><dfn noexport><code>histogram-index</code></dfn></dt>
   <dd>
     Value of <a dict-member for=AttributionImpressionOptions>histogramIndex</a>,
@@ -1734,6 +1742,8 @@ To <dfn noexport>parse a `Save-Impression` header</dfn> given a [=header value=]
 1.  Let |histogramIndex| be |dict|["<code>[=save-impression/histogram-index=]</code>"].
 1.  Let |conversionSites| be |dict|["<code>[=save-impression/conversion-sites=]</code>"]
     [=map/with default=] an empty [=structured header/inner list=].
+1.  Let |conversionCallers| be |dict|["<code>[=save-impression/conversion-callers=]</code>"]
+    [=map/with default=] an empty [=structured header/inner list=].
 1.  If any of |conversionSites|' [=list/items=] is not a [=structured header/string=], return an error.
 1.  Let |matchValue| be |dict|["<code>[=save-impression/match-value=]</code>"] [=map/with default=] 0.
 1.  If |matchValue| is not an [=structured header/integer=] or is less than 0, return an error.
@@ -1746,6 +1756,8 @@ To <dfn noexport>parse a `Save-Impression` header</dfn> given a [=header value=]
      :: |matchValue|
      : {{AttributionImpressionOptions/conversionSites}}
      :: |conversionSites|
+     : {{AttributionImpressionOptions/conversionCallers}}
+     :: |conversionCallers|
      : {{AttributionImpressionOptions/lifetimeDays}}
      :: |lifetimeDays|