Add some basic test coverage for layers.

Add a basic pixel test for z-index, transforms and clip-path.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/884503003

tests/lowlevel/layers-expected.sky

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

tests/lowlevel/layers.sky

+<sky>
+
+<import src="../resources/run-after-display.sky" as="runAfterDisplay" />
+<style>
+  sky {
+    padding: 10px;
+  }
+  div {
+    width: 50px;
+    height: 50px;
+    white-space: nowrap;
+  }
+</style>
+Transforms
+<div style="transform: translate3d(10px, 10px, 0); background-color: pink;">
+  One
+  <div style="transform: translate3d(10px, 10px, 0); background-color: salmon;">
+    Two
+    <div style="transform: translate3d(10px, 10px, 0); background-color: papayawhip;">
+      Three
+    </div>
+  </div>
+</div>
+
+<div style="position: absolute; right: 50px;">
+  <div style="background-color: red;">
+    1
+  </div>
+  <div style="position: absolute; left: 40px; top: 40px; z-index: 2; background-color: pink;">
+    4
+  </div>
+  <div style="position: absolute; left: 20px; top: 20px; z-index: 0; background-color: salmon;">
+    2
+  </div>
+  <div style="position: absolute; left: 35px; top: 20px; z-index: 1; background-color: papayawhip;">
+    3
+  </div>
+  <div style="background-color: green;">
+    1
+  </div>
+  z-index
+</div>
+
+<div style="position: absolute; bottom: 50px;">
+  Clip-path
+  <div style="background-color: pink; -webkit-clip-path: circle(25px at 25px 25px);">
+    <div style="background-color: salmon; -webkit-clip-path: circle(25px at 35px 35px);">
+      <div style="background-color: papayawhip; -webkit-clip-path: circle(25px at 45px 45px);"></div>
+    </div>
+  </div>
+</div>
+
+<script>
+runAfterDisplay(function() {
+  internals.notifyTestComplete("");
+});
+</script>
+
+</sky>