Bug 1662800 - Make the WebRenderScrollData dump nicer. r=botond

This makes the WebRenderScrollData dump more analogous to the layer tree dump,
in that it prints the layer entries one per line showing in-order nested tree
structure. It also omits printing things if they're not important.

Differential Revision: https://phabricator.services.mozilla.com/D96312

gfx/layers/wr/WebRenderLayerManager.cpp

@@ -377,7 +377,9 @@ void WebRenderLayerManager::EndTransactionWithoutLayer(
     }
     mScrollData.SetPaintSequenceNumber(mPaintSequenceNumber);
     if (dumpEnabled) {
-      mScrollData.Dump();
+      std::stringstream str;
+      str << mScrollData;
+      print_stderr(str);
     }
   }