Fix the bug that negative outline-offset is covered up by composited

third_party/WebKit/LayoutTests/compositing/overflow/overflow-scroll-with-negative-offset-translucent-outline-expected.html

+<script>
+onload = function() {
+    document.getElementById('scroller').scrollTop = 200;
+}
+</script>
+<style>
+#scroller {
+    background: local grey;
+    overflow: scroll;
+    width: 200px;
+    height: 200px;
+    box-sizing: border-box;
+}
+
+#scroller-outline {
+    will-change: transform;
+    outline: 5px solid rgba(0, 255, 0, 0.5);
+    outline-offset: -4px;
+    position: absolute;
+    width: 200px;
+    height: 200px;
+}
+
+.spacer {
+    height: 300px;
+}
+</style>
+<!-- #scroller outline should not be covered up by scrolling content background.
+    It is achieved by painting the outline into the decoration layer which draws
+    on top of all other layers. -->
+<div id="scroller-outline"></div>
+<div id="scroller">
+    <div class="spacer"></div>
+</div>
+