Address review comment

Author: cabanier

index.bs

@@ -1574,6 +1574,7 @@ enum XREye {
 
 [SecureContext, Exposed=Window] interface XRView {
   readonly attribute XREye eye;
+  readonly attribute unsigned long index;
   readonly attribute double? recommendedViewportScale;
 
   undefined requestViewportScale(double? scale);
@@ -1586,6 +1587,8 @@ The {{XRViewGeometry/transform}} is given in it's [=view/reference space=].
 
 The <dfn attribute for="XRView">eye</dfn> attribute describes the [=view/eye=] of the underlying [=view=]. This attribute's primary purpose is to ensure that pre-rendered stereo content can present the correct portion of the content to the correct eye.
 
+The <dfn attribute for="XRView">index</dfn> attribute describes a unique offet of this {{XRView}}. This MUST be used by the experience to identify the {{XRView}} for each {{XREye}}.
+
 The optional <dfn attribute for="XRView">recommendedViewportScale</dfn> attribute contains a UA-recommended viewport scale value that the application can use for a {{XRView/requestViewportScale()}} call to configure dynamic viewport scaling. It is `null` if the system does not implement a heuristic or method for determining a recommended scale. If not null, the value MUST be a numeric value greater than 0.0 and less than or equal to 1.0, and MUST be [=quantization|quantized=] to avoid providing detailed performance or GPU utilization data.
 
 Note: It is suggested to quantize the recommended viewport scale by rounding it to the nearest value from a short list of possible scale values, and using hysteresis to avoid instant changes when close to a boundary value. (This also helps avoid rapidly oscillating scale values which can be visually distracting or uncomfortable.)
@@ -2602,6 +2605,7 @@ interface XRVisibilityMaskChangeEvent : Event {
   constructor(DOMString type, XRVisibilityMaskChangeEventInit eventInitDict);
   [SameObject] readonly attribute XRSession session;
   readonly attribute XREye eye;
+  readonly attribute unsigned long index;
   [SameObject] readonly attribute Float32Array vertices;
   [SameObject] readonly attribute Uint32Array indices;
 };
@@ -2618,6 +2622,8 @@ The <dfn attribute for="XRVisibilityMaskChangeEvent">session</dfn> attribute ind
 
 The <dfn attribute for="XRVisibilityMaskChangeEvent">eye</dfn> attribute indicates which {{XREye}} the mask applies to.
 
+The <dfn attribute for="XRVisibilityMaskChangeEvent">index</dfn> attribute indicates which {{XRView/index}} the mask applies to.
+
 The <dfn attribute for="XRVisibilityMaskChangeEvent">vertices</dfn> attribute is a [=/list=] of <code>X</code>, <code>Y</code> coordinates. The experience MUST assume that the <code>Z</code> coordinate is <code>-1</code>. Each <code>X</code>, <code>Y</code>, <code>Z</code> coordinate describes a vertex. 
 If this array is empty, the whole region of the {{XRView}} SHOULD be drawn.