DevTools: fix internal error on updating unwired element tree

The main ElementsTreeOutline in ElementsPanel is wired to DOMModel
updates, but ETOutlines in Console / elsewhere are not.

In the unwired case, nodes in the tree may have null `children()`.
This can result in internal errors.

Bug: 896445
Change-Id: Ie903fdddb76b31a5460ce2dd4b753be7ba7eeb56
Reviewed-on: https://chromium-review.googlesource.com/c/1286860Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Erik Luo <luoe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607419}

third_party/WebKit/LayoutTests/http/tests/devtools/console/console-expand-removed-node-expected.txt

+Tests that removed Elements logged in the Console are properly formatted.
+
+Adding element
+console-expand-removed-node.js:19 <div>…</div>
+Removing element
+Expanding element in Console
+console-expand-removed-node.js:19 <div></div>
+

third_party/WebKit/LayoutTests/http/tests/devtools/console/console-expand-removed-node.js

+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+(async function() {
+  TestRunner.addResult(`Tests that removed Elements logged in the Console are properly formatted.\n`);
+  await TestRunner.loadModule('console_test_runner');
+  await TestRunner.showPanel('console');
+
+  TestRunner.addResult(`Adding element`);
+  await TestRunner.evaluateInPagePromise(`
+    var el = document.createElement('div');
+    var child = document.createElement('span');
+    el.appendChild(child);
+    document.body.appendChild(el);
+    undefined;
+  `);
+  const nodePromise = TestRunner.addSnifferPromise(Console.ConsoleViewMessage.prototype, '_formattedParameterAsNodeForTest');
+  TestRunner.evaluateInPagePromise(`console.log(el)`);
+  await nodePromise;
+  await ConsoleTestRunner.waitForPendingViewportUpdates();
+  ConsoleTestRunner.dumpConsoleMessages();
+
+  TestRunner.addResult(`Removing element`);
+  await TestRunner.evaluateInPagePromise(`el.remove()`);
+
+  TestRunner.addResult(`Expanding element in Console`);
+  await ConsoleTestRunner.expandConsoleMessagesPromise();
+  await ConsoleTestRunner.waitForRemoteObjectsConsoleMessagesPromise();
+  ConsoleTestRunner.dumpConsoleMessages();
+
+  TestRunner.completeTest();
+})();

third_party/blink/renderer/devtools/front_end/elements/ElementsTreeOutline.js

@@ -1229,7 +1229,8 @@ Elements.ElementsTreeOutline = class extends UI.TreeOutline {
       visibleChildren.push(beforePseudoElement);
 
     if (node.childNodeCount()) {
-      let children = node.children();
+      // Children may be stale when the outline is not wired to receive DOMModel updates.
+      let children = node.children() || [];
       if (!this._showHTMLCommentsSetting.get())
         children = children.filter(n => n.nodeType() !== Node.COMMENT_NODE);
       visibleChildren = visibleChildren.concat(children);