Paint local background colors onto foreground layer.

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

+<style>
+#scroller {
+    background: gray local content-box;
+    border: 10px solid rgba(0, 255, 0, 0.5);
+    overflow: scroll;
+    padding: 10px;
+    width: 200px;
+    height: 200px;
+    will-change: transform;
+}
+
+.child {
+    width: 30px;
+    height: 30px;
+    padding: 10px;
+    border: 10px solid green;
+    background: green content-box;
+}
+
+.spacer {
+    height: 300px;
+}
+</style>
+<!-- When the #scroller is composited, its local attached background will be
+    drawn into the composited layer as well as the background of its non
+    composited .child. This tests that the .child background is drawn
+    correctly to its bounds as they do include borders unlike #scroller. -->
+<div id="scroller">
+    <div class="child"></div>
+    <div class="spacer"></div>
+</div>