mus: Fix a teardown crash.

services/ui/ws/frame_generator.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "services/ui/ws/frame_generator.h"
 
 #include "base/containers/adapters.h"
 #include "cc/output/compositor_frame.h"
 #include "cc/quads/render_pass.h"
 #include "cc/quads/render_pass_draw_quad.h"
@@ skipping 239 matching lines @@
                  << surface_id.ToString();
       return;
     }
     SurfaceDependency dependency = {
         surface_id.local_frame_id(),
         surface_sequence_generator_.CreateSurfaceSequence()};
     surface->AddDestructionDependency(dependency.sequence);
     dependencies_[surface_id.frame_sink_id()] = dependency;
     // Observe |window_surface|'s window so that we can release references when
     // the window is destroyed.
-    if (!window_surface->window()->HasObserver(this))
-      window_surface->window()->AddObserver(this);
+    Add(window_surface->window());
     return;
   }
 
   // We are already holding a reference to this surface so there's no work to do
   // here.
   if (surface_id.local_frame_id() == it->second.local_frame_id)
     return;
 
   // If we have have an existing reference to a surface from the given
   // FrameSink, then we should release the reference, and then add this new
@@ skipping 24 matching lines @@
 void FrameGenerator::ReleaseAllSurfaceReferences() {
   cc::SurfaceManager* surface_manager = display_compositor_->manager();
   std::vector<uint32_t> sequences;
   for (auto& dependency : dependencies_)
     sequences.push_back(dependency.second.sequence.sequence);
   surface_manager->DidSatisfySequences(frame_sink_id_, &sequences);
   dependencies_.clear();
 }
 
 void FrameGenerator::OnWindowDestroying(ServerWindow* window) {
-  window->RemoveObserver(this);
+  Remove(window);
   ServerWindowCompositorFrameSinkManager* surface_manager =
       window->compositor_frame_sink_manager();
   // If FrameGenerator was observing |window|, then that means it had a surface
   // at some point in time and should have a
   // ServerWindowCompositorFrameSinkManager.
   DCHECK(surface_manager);
   ServerWindowCompositorFrameSink* default_compositor_frame_sink =
       surface_manager->GetDefaultCompositorFrameSink();
   if (default_compositor_frame_sink)
     ReleaseFrameSinkReference(default_compositor_frame_sink->frame_sink_id());
   ServerWindowCompositorFrameSink* underlay_compositor_frame_sink =
       surface_manager->GetUnderlayCompositorFrameSink();
   if (underlay_compositor_frame_sink)
     ReleaseFrameSinkReference(underlay_compositor_frame_sink->frame_sink_id());
 }
 
 }  // namespace ws
 
 }  // namespace ui