Revert "Disable TabManagerTest.ProactiveFastShutdownWithBeforeunloadHandler"

This reverts commit cb6c4c2d.

Reason for revert: Reverted suspected culprit; checking to see if still flaky.

Original change's description:
> Disable TabManagerTest.ProactiveFastShutdownWithBeforeunloadHandler
> 
> TBR=wjmaclean@chromium.org
> NOTRY=true
> 
> Bug: 874915
> Change-Id: Id0fae86ef2e130185d92ad28383c74036dcd96f3
> Reviewed-on: https://chromium-review.googlesource.com/1177881
> Reviewed-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}

TBR=xidachen@chromium.org,wjmaclean@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 874915
Change-Id: Ie5e977ce5893b35b7510e19c330b668ee6ff52f0
Reviewed-on: https://chromium-review.googlesource.com/1179348Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: James MacLean <wjmaclean@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584202}

chrome/browser/resource_coordinator/tab_manager_browsertest.cc

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