Convert CallbackList::Subscription to a standalone class.

Bug: 1103086
AX-Relnotes: n/a.
TBR: pinkerton
Change-Id: I3b241eb7234727f314dd85d1bdbb3a41ceca5938
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2522860
Reviewed-by: Peter Kasting <[email protected]>
Reviewed-by: Daniel Cheng <[email protected]>
Reviewed-by: danakj <[email protected]>
Commit-Queue: Peter Kasting <[email protected]>
Auto-Submit: Peter Kasting <[email protected]>
Cr-Commit-Position: refs/heads/master@{#830015}
diff --git a/content/browser/host_zoom_map_impl.h b/content/browser/host_zoom_map_impl.h
index 6fe489bf..fb2d6c1 100644
--- a/content/browser/host_zoom_map_impl.h
+++ b/content/browser/host_zoom_map_impl.h
@@ -54,7 +54,7 @@
                                int render_view_id) override;
   double GetDefaultZoomLevel() override;
   void SetDefaultZoomLevel(double level) override;
-  std::unique_ptr<Subscription> AddZoomLevelChangedCallback(
+  base::CallbackListSubscription AddZoomLevelChangedCallback(
       ZoomLevelChangedCallback callback) override;
 
   // Returns the current zoom level for the specified WebContents. This may