Bug 1241602 - Adds telemetry for MozMousePixelScroll usage. r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D106578

dom/base/UseCounters.conf

@@ -146,8 +146,9 @@ method MediaDevices.getDisplayMedia
 // Non-standard Document.mozSetImageElement.
 method Document.mozSetImageElement
 
-// Non-standard Window.DOMMouseScroll
+// Non-standard Window.DOMMouseScroll and MozMousePixelScroll
 custom ondommousescroll sets DOMMouseScroll event listener
+custom onmozmousepixelscroll sets MozMousePixelScroll event listener
 
 // Missing-property use counters.  We claim these are "method" use
 // counters, because we don't need a separate description string for

dom/events/EventListenerManager.cpp

@@ -419,6 +419,12 @@ void EventListenerManager::AddEventListenerInternal(
         doc->SetUseCounter(eUseCounter_custom_ondommousescroll);
       }
     }
+  } else if (aTypeAtom == nsGkAtoms::onMozMousePixelScroll) {
+    if (nsPIDOMWindowInner* window = GetInnerWindowForTarget()) {
+      if (Document* doc = window->GetExtantDoc()) {
+        doc->SetUseCounter(eUseCounter_custom_onmozmousepixelscroll);
+      }
+    }
   }
 
   if (IsApzAwareListener(listener)) {