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.html

+<style>
+#scroller {
+    outline: 5px solid rgba(0, 255, 0, 0.5);
+    outline-offset: -4px;
+    overflow: scroll;
+    width: 200px;
+    height: 200px;
+    box-sizing: border-box;
+    will-change: transform;
+}
+
+.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">
+    <div class="spacer"></div>
+</div>
+