[resize-observer] ResizeObservation constructor needs a ResizeObserverBoxOptions

Fixes w3c#7547, w3c#7548

Author: Loirooriol

resize-observer-1/Overview.bs

@@ -216,9 +216,11 @@ interface ResizeObserver {
 
         1. If |target| is in {{ResizeObserver/[[observationTargets]]}} slot, call {{ResizeObserver/unobserve()}} with argument <var>target</var>.
 
-        2. Let |resizeObservation| be new {{ResizeObservation}}(<var>target</var>, <var>options</var>).
+        2. Let |observedBox| be the value of the {{ResizeObserverOptions/box}} dictionary member of <var>options</var>.
 
-        3. Add the |resizeObservation| to the {{ResizeObserver/[[observationTargets]]}} slot.
+        3. Let |resizeObservation| be new {{ResizeObservation}}(<var>target</var>, <var>observedBox</var>).
+
+        4. Add the |resizeObservation| to the {{ResizeObserver/[[observationTargets]]}} slot.
 
     :  <dfn method for="ResizeObserver">unobserve(|target|)</dfn>
     ::
@@ -309,7 +311,7 @@ interface is not visible to Javascript.</p>
 
 <pre class="idl extract">
 interface ResizeObservation {
-    constructor(Element target);
+    constructor(Element target, ResizeObserverBoxOptions observedBox);
     readonly attribute Element target;
     readonly attribute ResizeObserverBoxOptions observedBox;
     readonly attribute FrozenArray&lt;ResizeObserverSize> lastReportedSizes;