Bug 1568950 - Part 2: Record use counters for SVG image when the image is going to be discarded; r=smaug

Part-1 patch (moving ReportUseCounters call to Document::Destroy) doesn't help SVG image case because the SVGDocumentWrapper::DestroyViewer() still happens too late.

Differential Revision: https://phabricator.services.mozilla.com/D42368

--HG--
extra : moz-landing-system : lando

Author: EdgarChen

image/VectorImage.cpp

@@ -1283,6 +1283,10 @@ VectorImage::RequestDiscard() {
     mProgressTracker->OnDiscard();
   }
 
+  if (Document* doc = mSVGDocumentWrapper->GetDocument()) {
+    doc->ReportUseCounters();
+  }
+
   return NS_OK;
 }