MD History: Improve test helpers, update synced tabs tests to use them

chrome/browser/resources/md_history/app.crisper.js

Index: chrome/browser/resources/md_history/app.crisper.js
diff --git a/chrome/browser/resources/md_history/app.crisper.js b/chrome/browser/resources/md_history/app.crisper.js
index 3e75862e14094e32974a703a7d4959977c019372..13564d27c06fb4aac553262553bc4e13af51e2f7 100644
--- a/chrome/browser/resources/md_history/app.crisper.js
+++ b/chrome/browser/resources/md_history/app.crisper.js
@@ -7745,6 +7745,16 @@ Polymer({
   is: 'cr-dialog',
   extends: 'dialog',
 
+  /** @override */
+  created: function() {
+    // If the active history entry changes (i.e. user clicks back button),
+    // all open dialogs should be cancelled.
+    window.addEventListener('popstate', function() {
+      if (this.open)
+        this.cancel();
+    }.bind(this));
+  },
+
   cancel: function() {
     this.fire('cancel');
     HTMLDialogElement.prototype.close.call(this, '');
@@ -14434,7 +14444,9 @@ Polymer({
   },
 
   unselectAllItems: function(count) {
-    this.getSelectedList_().unselectAllItems(count);
+    var selectedList = this.getSelectedList_();
+    if (selectedList)
+      selectedList.unselectAllItems(count);
   },
 
   /**