DevTools: do not highlight selected node on DevTools focus

Switching windows or using the Element picker may focus the DevTools
area with relatedTarget=null. In these cases, re-applying node
highlight is not needed.

Bug: 905252
Change-Id: Ie3ae228c9cc404f16598ff428d9e593bb90e33e5
Reviewed-on: https://chromium-review.googlesource.com/c/1336910
Commit-Queue: Andrey Lushnikov <lushnikov@chromium.org>
Reviewed-by: Andrey Lushnikov <lushnikov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608541}

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

@@ -550,7 +550,8 @@ Elements.ElementsTreeOutline = class extends UI.TreeOutline {
     const listItem = event.target.enclosingNodeOrSelfWithNodeName('li');
     if (!listItem || !listItem.treeElement || !listItem.treeElement.selected)
       return;
-    this._highlightTreeElement(/** @type {!UI.TreeElement} */ (listItem.treeElement), true /* showInfo */);
+    if (event.relatedTarget)
+      this._highlightTreeElement(/** @type {!UI.TreeElement} */ (listItem.treeElement), true /* showInfo */);
   }
 
   /**