[MD History] Reenable SyncedTabsTest.

This CL skips the focus test that is likely the cause of failures and
reenables the test on all platforms.

BUG=640862

Review-Url: https://codereview.chromium.org/2582543005
Cr-Commit-Position: refs/heads/master@{#439056}

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'

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

@@ -214,8 +214,7 @@ MaterialHistorySyncedTabsTest.prototype = {
   ]),
 };
 
-// Fails on Mac, http://crbug.com/640862
-TEST_F('MaterialHistorySyncedTabsTest', 'DISABLED_All', function() {
+TEST_F('MaterialHistorySyncedTabsTest', 'All', function() {
   mocha.run();
 });