[chromedriver] Add shadow DOM support to chromedriver.

chrome/test/chromedriver/js/focus.js

 // Copyright (c) 2013 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.
 
 function focus(element) {
   // Focus the target element in order to send keys to it.
   // First, the currently active element is blurred, if it is different from
   // the target element. We do not want to blur an element unnecessarily,
   // because this may cause us to lose the current cursor position in the
   // element.
@@ skipping 20 matching lines @@
   if (element != prevActiveElement && element.value &&
       element.value.length && element.setSelectionRange) {
     try {
       element.setSelectionRange(element.value.length, element.value.length);
     } catch (error) {
       if (!(error instanceof TypeError) && !(error instanceof DOMException &&
           error.code == DOMException.INVALID_STATE_ERR))
         throw error;
     }
   }
-  if (element != doc.activeElement)
+
+  var activeElement = doc.activeElement;
+  // If the element is in a shadow DOM, then as far as the document is
+  // concerned, the shadow host is the active element. We need to go through the
+  // tree of shadow DOMs to check that the element we gave focus to is now
+  // active.
+  if (element != activeElement) {
+    var shadowRoot = activeElement.shadowRoot;
+    while (shadowRoot) {
+      var activeElement = shadowRoot.activeElement;
+      if (element == activeElement) {
+        // the shadow DOM's active element is our requested element. We're good.
+        break;
+      }
+      // The shadow DOM's active element isn't our requested element, check to
+      // see if there's a nested shadow DOM.
+      shadowRoot = activeElement.shadowRoot;
+    }
+  }
+  if (element != activeElement)
     throw new Error('cannot focus element');
 }