Pepper takes ownership of a mailbox before passing it to the texture layer.

content/renderer/pepper/ppb_graphics_3d_impl.h

 // Copyright (c) 2012 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.
 
 #ifndef CONTENT_RENDERER_PEPPER_PPB_GRAPHICS_3D_IMPL_H_
 #define CONTENT_RENDERER_PEPPER_PPB_GRAPHICS_3D_IMPL_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 42 matching lines @@
   // Returns true if binding/unbinding is successful.
   bool BindToInstance(bool bind);
 
   // Returns true if the backing texture is always opaque.
   bool IsOpaque();
 
   // Notifications about the view's progress painting.  See PluginInstance.
   // These messages are used to send Flush callbacks to the plugin.
   void ViewInitiatedPaint();
 
-  void GetBackingMailbox(gpu::Mailbox* mailbox, gpu::SyncToken* sync_token) {
-    *mailbox = mailbox_;
-    *sync_token = sync_token_;
-  }
+  bool TakeBackingMailbox(gpu::Mailbox* mailbox, gpu::SyncToken* sync_token);
+  void ReturnBackingMailbox(const gpu::Mailbox& mailbox,
+                            const gpu::SyncToken& sync_token,
+                            bool is_lost);
 
   gpu::CommandBufferProxyImpl* GetCommandBufferProxy();
 
   gpu::GpuChannelHost* channel() { return channel_.get(); }
 
  protected:
   ~PPB_Graphics3D_Impl() override;
   // ppapi::PPB_Graphics3D_Shared overrides.
   gpu::CommandBuffer* GetCommandBuffer() override;
   gpu::GpuControl* GetGpuControl() override;
@@ skipping 19 matching lines @@
 
   // True if context is bound to instance.
   bool bound_to_instance_;
   // True when waiting for compositor to commit our backing texture.
   bool commit_pending_;
 
 #if DCHECK_IS_ON()
   bool lost_context_ = false;
 #endif
 
-  gpu::Mailbox mailbox_;
-  gpu::SyncToken sync_token_;
   bool has_alpha_;
   scoped_refptr<gpu::GpuChannelHost> channel_;
   std::unique_ptr<gpu::CommandBufferProxyImpl> command_buffer_;
 
   base::WeakPtrFactory<PPB_Graphics3D_Impl> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(PPB_Graphics3D_Impl);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_RENDERER_PEPPER_PPB_GRAPHICS_3D_IMPL_H_