[MD History] Add shift-selection to the history lists.

chrome/test/data/webui/md_history/test_util.js

 // Copyright 2016 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.
 
 /*
  * Waits for queued up tasks to finish before proceeding. Inspired by:
  * https://github.com/Polymer/web-component-tester/blob/master/browser/environment/helpers.js#L97
  */
 function flush() {
   Polymer.dom.flush();
@@ skipping 91 matching lines @@
       if (!predicate(e))
         return;
 
       resolve();
       element.removeEventListener(eventName, listener);
     }
 
     element.addEventListener(eventName, listener);
   });
 }
+
+/**
+ * Sends a shift click event to |element|.
+ * @param {HTMLElement} element
+ */
+function shiftClick(element) {
+  var xy = MockInteractions.middleOfNode(element);
+  var props = {
+    bubbles: true,
+    cancelable: true,
+    clientX: xy.x,
+    clientY: xy.y,
+    buttons: 1,
+    shiftKey: true,
+  };
+
+  element.dispatchEvent(new MouseEvent('mousedown', props));
+  element.dispatchEvent(new MouseEvent('mouseup', props));
+  element.dispatchEvent(new MouseEvent('click', props));
+}