Mark NewTabUITest.NTPHasThumbnails FLAKY.

chrome/browser/dom_ui/new_tab_ui_uitest.cc

Index: chrome/browser/dom_ui/new_tab_ui_uitest.cc
diff --git a/chrome/browser/dom_ui/new_tab_ui_uitest.cc b/chrome/browser/dom_ui/new_tab_ui_uitest.cc
index 563581114f300e6c51c7156ec0e6f6ec4d5e0a8d..6b3fdc440c27c7bd03a3a2913092808a89c28bc8 100644
--- a/chrome/browser/dom_ui/new_tab_ui_uitest.cc
+++ b/chrome/browser/dom_ui/new_tab_ui_uitest.cc
@@ -30,7 +30,8 @@ class NewTabUITest : public UITest {
   }
 };
 
-TEST_F(NewTabUITest, NTPHasThumbnails) {
+// Fails on XP, Linux: http://crbug.com/51721
+TEST_F(NewTabUITest, FLAKY_NTPHasThumbnails) {
   // Switch to the "new tab" tab, which should be any new tab after the
   // first (the first is about:blank).
   scoped_refptr<BrowserProxy> window(automation()->GetBrowserWindow(0));