Add opacity to the layers.sky test.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/883903003

engine/src/flutter/tests/lowlevel/layers-expected.sky

@@ -1,7 +1,7 @@
 <foo>
 <import src="../resources/run-after-display.sky" as="runAfterDisplay" />
 
-<img src="http://storage.googleapis.com/mojo/sky-pngs/d888b5c2e16523f2acfba693cfe42b7f142c4076" />
+<img src="http://storage.googleapis.com/mojo/sky-pngs/ed3514fd579dbaa8c18594cf99f3b60bf4b6ee75" />
 
 <script>
 window.addEventListener('load', function() {

engine/src/flutter/tests/lowlevel/layers.sky

@@ -22,6 +22,23 @@ Transforms
   </div>
 </div>
 
+<div style="position: absolute; top: 250px;">
+  Opacity
+  <div style="opacity: 0.8; background-color: pink;">
+    One
+    <div style="opacity: 0.7; position: absolute; left: 10px; background-color: salmon;">
+      Two
+      <div style="opacity: 0.6; position: absolute; left: 20px; background-color: papayawhip;">
+        Three
+        <div style="opacity: 0; transform: translate3d(0, 10px, 0); background-color: papayawhip;">
+          Clear
+        </div>
+      </div>
+    </div>
+  </div>
+</div>
+
+
 <div style="position: absolute; right: 50px;">
   <div style="background-color: red;">
     1