bug 1442980, add basic test for print previewed shadow DOM, r=bdahl

--HG--
extra : rebase_source : 66d28f5566710346022ef9a8f8d3459579edb546

layout/base/tests/chrome/printpreview_helper.xul

@@ -259,6 +259,28 @@ function runTest6end() {
   printpreview();
   exitprintpreview();
 
+  requestAnimationFrame(function() { setTimeout(runTest7); } );
+}
+
+function runTest7() {
+  var contentText = "<a href='#'>mozilla</a><input>test<select><option>option1</option></select>";
+  // Create normal content
+  window.frames[0].document.body.innerHTML =
+    "<div>" + contentText + "</div>";
+  window.frames[0].document.body.firstChild.value =
+    window.frames[0].document.body.firstChild.getAttribute('value');
+  printpreview();
+  ctx1.drawWindow(window.frames[1], 0, 0, 400, 400, "rgb(255,255,255)");
+  exitprintpreview();
+
+  window.frames[0].document.body.innerHTML = "<div></div>";
+  var sr = window.frames[0].document.body.firstChild.attachShadow({mode: "open"});
+  sr.innerHTML = contentText;
+  printpreview();
+  ctx2.drawWindow(window.frames[1], 0, 0, 400, 400, "rgb(255,255,255)");
+  exitprintpreview();
+  ok(compareCanvases(), "Printing light DOM and shadow DOM should create same output");
+
   finish();
 }
 

layout/base/tests/chrome/test_printpreview.xul

@@ -11,6 +11,8 @@
 <script type="application/javascript">
 <![CDATA[
 SimpleTest.waitForExplicitFinish();
-window.open("printpreview_helper.xul", "printpreview", "chrome,width=100,height=100");
+SpecialPowers.pushPrefEnv({"set":[["dom.webcomponents.shadowdom.enabled", true]]}, function() {
+  window.open("printpreview_helper.xul", "printpreview", "chrome,width=100,height=100");
+});
 ]]></script>
 </window>