Add a ScopedNestableTaskAllower to Fix Tab Navigation

MacOS's alert dialog seems to need allowing nestable tasks for tab
navigation to work.

BUG=778562

Change-Id: Ic8654defa4e6ec1c7089639108e8a937e923b486
Reviewed-on: https://chromium-review.googlesource.com/747849
Commit-Queue: Robert Liao <robliao@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513270}

chrome/browser/ui/cocoa/extensions/extension_uninstall_dialog_cocoa.mm

@@ -9,6 +9,7 @@
 #include <string>
 
 #import "base/mac/scoped_nsobject.h"
+#include "base/message_loop/message_loop.h"
 #import "base/strings/sys_string_conversions.h"
 #include "base/strings/utf_string_conversions.h"
 #include "chrome/browser/ui/cocoa/browser_dialogs_views_mac.h"
@@ -69,7 +70,13 @@ void ExtensionUninstallDialogCocoa::Show() {
     [alert setAccessoryView:reportAbuseCheckbox];
   }
 
-  if ([alert runModal] == NSAlertFirstButtonReturn) {
+  NSModalResponse response;
+  {
+    base::MessageLoop::ScopedNestableTaskAllower allow_nested(
+        base::MessageLoop::current());
+    response = [alert runModal];
+  }
+  if (response == NSAlertFirstButtonReturn) {
     bool report_abuse_checked =
         reportAbuseCheckbox.get() && [reportAbuseCheckbox state] == NSOnState;
     OnDialogClosed(report_abuse_checked ?