Paint local background colors onto foreground layer.

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

+<script>
+onload = function() {
+    document.getElementById('scroller').scrollTo(0, 200);
+}
+</script>
+<style>
+#scroller {
+    background: gray local content-box;
+    border: 10px solid rgba(0, 255, 0, 0.5);
+    overflow: scroll;
+    padding: 10px;
+    width: 200px;
+    height: 200px;
+}
+
+.spacer {
+    height: 300px;
+}
+</style>
+<!-- #scroller has a locally attached background with padding so when it is
+    scrolled we can see the background slide up. Doing this on a composited
+    element without repainting requires the background to be painted into
+    the scrolling contents layer and correctly account for that layer not
+    including the border in its bounds. -->
+<div id="scroller">
+    <div class="spacer"></div>
+</div>