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

content/renderer/pepper/ppb_graphics_3d_impl.h

Index: content/renderer/pepper/ppb_graphics_3d_impl.h
diff --git a/content/renderer/pepper/ppb_graphics_3d_impl.h b/content/renderer/pepper/ppb_graphics_3d_impl.h
index b356b508c927c9c76ca0dd62ff4af2d2d45e4f1f..30a17d523e7e1ae054f87d95787a4327929eb3fd 100644
--- a/content/renderer/pepper/ppb_graphics_3d_impl.h
+++ b/content/renderer/pepper/ppb_graphics_3d_impl.h
@@ -60,10 +60,10 @@ class PPB_Graphics3D_Impl : public ppapi::PPB_Graphics3D_Shared,
   // 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();
 
@@ -103,8 +103,6 @@ class PPB_Graphics3D_Impl : public ppapi::PPB_Graphics3D_Shared,
   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_;