Add test for ash devtools DOM agent getDocument method

ash/common/devtools/ash_devtools_dom_agent.cc

Index: ash/common/devtools/ash_devtools_dom_agent.cc
diff --git a/ash/common/devtools/ash_devtools_dom_agent.cc b/ash/common/devtools/ash_devtools_dom_agent.cc
index 402125e81da9284619f6d39ad5b8ddd4ed171aef..ebb928589c6de9927ce987adebbb71e9e501abf2 100644
--- a/ash/common/devtools/ash_devtools_dom_agent.cc
+++ b/ash/common/devtools/ash_devtools_dom_agent.cc
@@ -21,14 +21,15 @@ std::unique_ptr<DOM::Node> BuildNode(
     std::unique_ptr<Array<DOM::Node>> children) {
   static DOM::NodeId node_ids = 0;
   constexpr int kDomElementNodeType = 1;
-  return DOM::Node::create()
-      .setNodeId(node_ids++)
-      .setNodeName(name)
-      .setNodeType(kDomElementNodeType)
-      .setAttributes(std::move(attributes))
-      .setChildNodeCount(children->length())
-      .setChildren(std::move(children))
-      .build();
+  std::unique_ptr<DOM::Node> node = DOM::Node::create()
+                                        .setNodeId(node_ids++)
+                                        .setNodeName(name)
+                                        .setNodeType(kDomElementNodeType)
+                                        .setAttributes(std::move(attributes))
+                                        .build();
+  node->setChildNodeCount(children->length());
+  node->setChildren(std::move(children));
+  return node;
 }
 
 std::unique_ptr<Array<std::string>> GetAttributes(const ash::WmWindow* window) {