Add a UA style, "display: contents", for <slot>

third_party/WebKit/LayoutTests/shadow-dom/slots.html

Index: third_party/WebKit/LayoutTests/shadow-dom/slots.html
diff --git a/third_party/WebKit/LayoutTests/shadow-dom/slots.html b/third_party/WebKit/LayoutTests/shadow-dom/slots.html
index c7412ed75468caa29bc02c5f1dbcb70e7cfd33e5..50bcfa9e92cd3a13c8873d3b31e8eb5b491d97d2 100644
--- a/third_party/WebKit/LayoutTests/shadow-dom/slots.html
+++ b/third_party/WebKit/LayoutTests/shadow-dom/slots.html
@@ -3,6 +3,8 @@
 <script src="../resources/testharnessreport.js"></script>
 <script src="resources/shadow-dom.js"></script>
 
+<slot id='slot_in_document'></slot>
+
 <div id="test_basic">
   <div id="host">
     <template data-mode="open">
@@ -14,6 +16,9 @@
 
 <script>
 test(() => {
+  assert_equals(window.getComputedStyle(slot_in_document).getPropertyValue('display'),
+                'contents');
+
   let n = createTestTree(test_basic);
   removeWhiteSpaceOnlyTextNodes(n.test_basic);