Fixed crash in the fast unload controller browser tests.

This CL is consist of two patches:
1. Turn the unload controller tests on for fast unload controller.
This patchset (the file unload_browsertest.cc only) returns back
the functionality reviewed and merged on
https://chromium-review.googlesource.com/c/chromium/src/+/1012118
and reverted on
https://chromium-review.googlesource.com/c/chromium/src/+/1022553

2. Fixed crash in the fast unload controller browser tests.
The original CL was reverted because of that crash. The reason of the crash is,
the callback resets itself. The same issue has already been fixed in
the unload controller (CL https://codereview.chromium.org/471763008 ),
but not applied to the fast unload controller.
This patchset (the file fast_unload_controller.cc only) brings the missed fix.

Change-Id: I9d867131897457efad0c96bc7d2fe0ffa2a65729
Reviewed-on: https://chromium-review.googlesource.com/1025714Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554780}

chrome/browser/ui/fast_unload_controller.cc

@@ -428,8 +428,14 @@ void FastUnloadController::ProcessPendingTabs(bool skip_beforeunload) {
   }
 
   if (is_calling_before_unload_handlers()) {
+    base::OnceCallback<void(bool)> on_close_confirmed = on_close_confirmed_;
+    // Reset |on_close_confirmed_| in case the callback tests
+    // |is_calling_before_unload_handlers()|, we want to return that calling
+    // is complete.
+    if (tabs_needing_unload_.empty())
+      on_close_confirmed_.Reset();
     if (!skip_beforeunload)
-      on_close_confirmed_.Run(true);
+      std::move(on_close_confirmed).Run(true);
     return;
   }