DevTools: Don't focus layers panel when it is shown

The layers panel ends up stealing focus and disrupting keyboard navigation.

BUG=706699

Review-Url: https://codereview.chromium.org/2819863004
Cr-Commit-Position: refs/heads/master@{#468155}

third_party/WebKit/Source/devtools/front_end/layers/LayersPanel.js

@@ -81,7 +81,6 @@ Layers.LayersPanel = class extends UI.PanelWithSidebar {
     super.wasShown();
     if (this._model)
       this._model.enable();
-    this._layerTreeOutline.focus();
   }
 
   /**