[MD History] Reenable SyncedTabsTest.

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

Index: chrome/test/data/webui/md_history/history_synced_tabs_test.js
diff --git a/chrome/test/data/webui/md_history/history_synced_tabs_test.js b/chrome/test/data/webui/md_history/history_synced_tabs_test.js
index e7949b0d78839bd690a035ead3af21c2fe42bd2b..69ecaf59f6edf6b158e0f4a1db4aab02756ba89c 100644
--- a/chrome/test/data/webui/md_history/history_synced_tabs_test.js
+++ b/chrome/test/data/webui/md_history/history_synced_tabs_test.js
@@ -225,7 +225,8 @@ suite('<history-synced-device-manager>', function() {
     });
   });
 
-  test('focus and keyboard nav', function() {
+  // Fails on Mac, http://crbug.com/640862
+  test.skip('focus and keyboard nav', function() {
     var sessionList = [
       createSession('Nexus 5', [createWindow([
                       'http://www.example.com', 'http://www.google.com'