history: add context menu buttons to the tab order.

Part #1 in fixing the synced devices menu.

R=dmazzoni@chromium.org
BUG=362424

Review URL: https://codereview.chromium.org/642223002

Cr-Commit-Position: refs/heads/master@{#299065}

chrome/browser/resources/history/other_devices.js

@@ -185,6 +185,7 @@ Device.prototype.getDOMNode = function(maxNumTabs, row) {
   heading.addEventListener('contextmenu', handleDropDownFocus);
 
   var dropDownButton = new cr.ui.ContextMenuButton;
+  dropDownButton.tabIndex = 0;
   dropDownButton.classList.add('drop-down');
   dropDownButton.addEventListener('mousedown', function(event) {
       handleDropDownFocus(event);