[MD History] Reenable SyncedTabsTest.

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

Index: chrome/test/data/webui/md_history/md_history_browsertest.js
diff --git a/chrome/test/data/webui/md_history/md_history_browsertest.js b/chrome/test/data/webui/md_history/md_history_browsertest.js
index 23de4b2931b54366efae74f0d4ac72ddd1facbf4..76186a893d889fe5f5b829eb299061a8c847c72c 100644
--- a/chrome/test/data/webui/md_history/md_history_browsertest.js
+++ b/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();
 });