WebUI: fix cr-action-menu keyboard off-by-one issue

BUG=None
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2803543003
Cr-Commit-Position: refs/heads/master@{#462732}

chrome/test/data/webui/cr_elements/cr_action_menu_test.js

@@ -40,6 +40,10 @@ suite('CrActionMenu', function() {
     MockInteractions.keyDownOn(menu, 'ArrowDown', [], 'ArrowDown');
   }
 
+  function up() {
+    MockInteractions.keyDownOn(menu, 'ArrowUp', [], 'ArrowUp');
+  }
+
   test('hidden or disabled items', function() {
     menu.showAt(document.querySelector('#dots'));
     down();
@@ -59,10 +63,6 @@ suite('CrActionMenu', function() {
   });
 
   test('focus after down/up arrow', function() {
-    function up() {
-      MockInteractions.keyDownOn(menu, 'ArrowUp', [], 'ArrowUp');
-    }
-
     menu.showAt(document.querySelector('#dots'));
 
     // The menu should be focused when shown, but not on any of the items.
@@ -93,6 +93,14 @@ suite('CrActionMenu', function() {
     assertEquals(items[0], menu.root.activeElement);
   });
 
+  test('pressing up arrow when no focus will focus last item', function(){
+    menu.showAt(document.querySelector('#dots'));
+    assertEquals(menu, document.activeElement);
+
+    up();
+    assertEquals(items[items.length - 1], menu.root.activeElement);
+  });
+
   test('close on resize', function() {
     menu.showAt(document.querySelector('#dots'));
     assertTrue(menu.open);

ui/webui/resources/cr_elements/cr_action_menu/cr_action_menu.js

@@ -131,6 +131,10 @@ Polymer({
     var focusedIndex =
         Array.prototype.indexOf.call(this.options_, this.root.activeElement);
 
+    // Handle case where nothing is focused and up is pressed.
+    if (focusedIndex === -1 && step === -1)
+      focusedIndex = 0;
+
     do {
       focusedIndex = (numOptions + focusedIndex + step) % numOptions;
       nextOption = this.options_[focusedIndex];