Disable TabManagerTest.ProactiveFastShutdownWithBeforeunloadHandler

TBR=wjmaclean@chromium.org
NOTRY=true

Bug: 874915
Change-Id: Id0fae86ef2e130185d92ad28383c74036dcd96f3
Reviewed-on: https://chromium-review.googlesource.com/1177881Reviewed-by: Xida Chen <xidachen@chromium.org>
Reviewed-by: James MacLean <wjmaclean@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583660}

chrome/browser/resource_coordinator/tab_manager_browsertest.cc

@@ -932,8 +932,9 @@ IN_PROC_BROWSER_TEST_F(TabManagerTest, UrgentFastShutdownWithUnloadHandler) {
 #endif  // OS_CHROMEOS
 }
 
+// https://crbug.com/874915, flaky on all platform
 IN_PROC_BROWSER_TEST_F(TabManagerTest,
-                       ProactiveFastShutdownWithBeforeunloadHandler) {
+                       DISABLED_ProactiveFastShutdownWithBeforeunloadHandler) {
   ASSERT_TRUE(embedded_test_server()->Start());
   // Disable the protection of recent tabs.
   OpenTwoTabs(GURL(chrome::kChromeUIAboutURL),