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

content/renderer/pepper/ppb_graphics_3d_impl.cc

Index: content/renderer/pepper/ppb_graphics_3d_impl.cc
diff --git a/content/renderer/pepper/ppb_graphics_3d_impl.cc b/content/renderer/pepper/ppb_graphics_3d_impl.cc
index 24c6513c12fb8119bb4d5ef6293d885dcc83d5e9..a98814db21b9538085765ea6ea5df58743e5bf12 100644
--- a/content/renderer/pepper/ppb_graphics_3d_impl.cc
+++ b/content/renderer/pepper/ppb_graphics_3d_impl.cc
@@ -143,8 +143,6 @@ gpu::GpuControl* PPB_Graphics3D_Impl::GetGpuControl() {
 
 int32_t PPB_Graphics3D_Impl::DoSwapBuffers(const gpu::SyncToken& sync_token) {
   DCHECK(command_buffer_);
-  if (sync_token.HasData())
-    sync_token_ = sync_token;
 
   if (bound_to_instance_) {
     // If we are bound to the instance, we need to ask the compositor
@@ -159,9 +157,8 @@ int32_t PPB_Graphics3D_Impl::DoSwapBuffers(const gpu::SyncToken& sync_token) {
   } else {
     // Wait for the command to complete on the GPU to allow for throttling.
     command_buffer_->SignalSyncToken(
-        sync_token_,
-        base::Bind(&PPB_Graphics3D_Impl::OnSwapBuffers,
-                   weak_ptr_factory_.GetWeakPtr()));
+        sync_token, base::Bind(&PPB_Graphics3D_Impl::OnSwapBuffers,
+                               weak_ptr_factory_.GetWeakPtr()));
   }
 
   return PP_OK_COMPLETIONPENDING;
@@ -262,13 +259,22 @@ bool PPB_Graphics3D_Impl::InitRaw(PPB_Graphics3D_API* share_context,
   if (command_buffer_id)
     *command_buffer_id = command_buffer_->GetCommandBufferID();
 
-  mailbox_ = gpu::Mailbox::Generate();
-  if (!command_buffer_->ProduceFrontBuffer(mailbox_))
-    return false;
+  return true;
+}
 
+bool PPB_Graphics3D_Impl::TakeBackingMailbox(gpu::Mailbox* mailbox,
+                                             gpu::SyncToken* sync_token) {
+  *mailbox = gpu::Mailbox::Generate();

[piman, 2016/04/26 01:54:01]

This is kind of costly (generate a crypto-secure number). Also, the
MailboxManager keeps a list of all mailboxes associated with a TextureRef, so
this would cause a logical leak. Could we keep a queue of these, recycling in
ReturnBackingMailbox?

[erikchen, 2016/04/27 16:31:23]

Done.

+  command_buffer_->TakeFrontBuffer(*mailbox, sync_token);
   return true;
 }
 
+void PPB_Graphics3D_Impl::ReturnBackingMailbox(const gpu::Mailbox& mailbox,
+                                               const gpu::SyncToken& sync_token,
+                                               bool is_lost) {
+  command_buffer_->ReturnFrontBuffer(mailbox, sync_token, is_lost);
+}
+
 void PPB_Graphics3D_Impl::OnGpuControlErrorMessage(const char* message,
                                                    int32_t id) {
   if (!bound_to_instance_)