Paint local background colors onto foreground layer.

third_party/WebKit/LayoutTests/compositing/overflow/overflow-scroll-with-transparent-background.html

+<script>
+if (window.testRunner)
+    window.testRunner.dumpAsText();
+
+function hasNotOpaqueCompositedScrollingContentsLayer(layer) {
+    if (layer.name == "Scrolling Contents Layer")
+        return !layer.contentsOpaque;
+    if (layer.children) {
+        for (var i = 0; i < layer.children.length; i++) {
+            if (hasNotOpaqueCompositedScrollingContentsLayer(layer.children[i]))
+                return true;
+        }
+    }
+    return false;
+}
+
+onload = function() {
+    if (!window.testRunner || !window.internals)
+        return;
+
+    var result = "";
+    result += "Should not have opaque composited scrolling contents layer: ";
+    if (hasNotOpaqueCompositedScrollingContentsLayer(JSON.parse(window.internals.layerTreeAsText(document))))
+        result += "Pass.\n";
+    else
+        result += "Fail.\n";
+    window.testRunner.setCustomTextOutput(result);
+}
+</script>
+<style>
+#scroller {
+    background: white local content-box;
+    border: 10px solid rgba(0, 255, 0, 0.5);
+    overflow: scroll;
+    padding: 10px;
+    width: 180px;
+    height: 180px;
+    will-change: transform;
+}
+.spacer {
+    height: 280px;
+}
+</style>
+<!-- The scroller's Scrolling Contents Layer should be transparent due to the
+    content-box clip with a 10px padding leaving a transparent gap between.
+-->
+<div id="scroller"><div class="spacer"></div></div>
+<pre id="layertree"></pre>