Add a missing include for ScopedObserver.

As is, it's getting a declaration out of an include in /components
someplace. I'm retiring ScopedObserver in /components, which breaks
compile here.

Bug: 1145565
Change-Id: I69a2dc15aaef4199c91c01177e0ebfcc22e2da24
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533140
Commit-Queue: David Roger <droger@chromium.org>
Auto-Submit: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826824}

ios/chrome/browser/ui/settings/clear_browsing_data/clear_browsing_data_manager.mm

@@ -7,6 +7,7 @@
 #include "base/bind.h"
 #include "base/mac/foundation_util.h"
 #include "base/metrics/histogram_macros.h"
+#include "base/scoped_observer.h"
 #include "base/strings/sys_string_conversions.h"
 #include "components/browsing_data/core/history_notice_utils.h"
 #include "components/browsing_data/core/pref_names.h"