Bug 1555048 - Ensure impression observer element is in correct spot (#5074)

content-src/components/DiscoveryStreamImpressionStats/_ImpressionStats.scss

@@ -1,5 +1,6 @@
 .impression-observer {
   position: absolute;
+  top: 0;
   width: 100%;
   height: 100%;
   pointer-events: none;