WebUI cr-dialog: Cancel dialog on any 'popstate' event.

ui/webui/resources/cr_elements/cr_dialog/cr_dialog.js

Index: ui/webui/resources/cr_elements/cr_dialog/cr_dialog.js
diff --git a/ui/webui/resources/cr_elements/cr_dialog/cr_dialog.js b/ui/webui/resources/cr_elements/cr_dialog/cr_dialog.js
index 5deb50cdea1c893c1fb7a77c852029fbd5397bd3..fc13095d5eec9c815c1c54e713b4f6206a0a0f11 100644
--- a/ui/webui/resources/cr_elements/cr_dialog/cr_dialog.js
+++ b/ui/webui/resources/cr_elements/cr_dialog/cr_dialog.js
@@ -15,6 +15,16 @@ 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, '');