Disable flaky BrowserCloseManagerBrowserTest.AddBeforeUnloadDuringClosing on Mac.

chrome/browser/lifetime/browser_close_manager_browsertest.cc

Index: chrome/browser/lifetime/browser_close_manager_browsertest.cc
diff --git a/chrome/browser/lifetime/browser_close_manager_browsertest.cc b/chrome/browser/lifetime/browser_close_manager_browsertest.cc
index 2e18057cb8eb55aebf01388a4dca0ec766cdea0f..121465666ff495d4c7074e75c38cc77f53d42843 100644
--- a/chrome/browser/lifetime/browser_close_manager_browsertest.cc
+++ b/chrome/browser/lifetime/browser_close_manager_browsertest.cc
@@ -744,8 +744,14 @@ IN_PROC_BROWSER_TEST_P(BrowserCloseManagerBrowserTest,
   EXPECT_TRUE(BrowserList::GetInstance()->empty());
 }
 
+#if defined(OS_MACOSX)
+// Flaky on Mac OSX. See http://crbug.com/700271.
+#define MAYBE_AddBeforeUnloadDuringClosing DISABLED_AddBeforeUnloadDuringClosing
+#else
+#define MAYBE_AddBeforeUnloadDuringClosing AddBeforeUnloadDuringClosing
+#endif
 IN_PROC_BROWSER_TEST_P(BrowserCloseManagerBrowserTest,
-                       AddBeforeUnloadDuringClosing) {
+                       MAYBE_AddBeforeUnloadDuringClosing) {
   // TODO(crbug.com/250305): Currently FastUnloadController is broken.
   // And it is difficult to fix this issue without fixing that one.
   if (GetParam())