Do not add destruction observer to PooledSharedImageVideoProvider

PooledSharedImageVideoProvider::GpuHelperImpl currently registers
a stub destruction observer in its constructor. It does not remove
this observer in its destructor, which can lead to a crash when
CommandBufferStub attempts to notify it of destruction.

I was going to unregister in destruction, but then noticed that
this observer isn't needed:
- CommandBufferHelper already registers its own observer internally
- GpuHelperImpl just used this notification to clear
  |command_buffer_helper_|, which is already taken care of internally
  in that class.

So instead this CL just removes the observer from GpuHelperImpl.

Bug: 1010612
Change-Id: I47d1982d6ccaf8624650a4274e8cb1500aba4f9b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1835168Reviewed-by: Frank Liberato <liberato@chromium.org>
Commit-Queue: Eric Karl <ericrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702204}

media/gpu/android/pooled_shared_image_video_provider.cc

@@ -188,7 +188,6 @@ PooledSharedImageVideoProvider::GpuHelperImpl::GpuHelperImpl(
     : weak_factory_(this) {
   gpu::CommandBufferStub* stub = get_stub_cb.Run();
   if (stub) {
-    stub->AddDestructionObserver(this);
     command_buffer_helper_ = CommandBufferHelper::Create(stub);
   }
 }
@@ -214,9 +213,4 @@ void PooledSharedImageVideoProvider::GpuHelperImpl::OnSyncTokenCleared(
   std::move(cb).Run();
 }
 
-void PooledSharedImageVideoProvider::GpuHelperImpl::OnWillDestroyStub(
-    bool have_context) {
-  command_buffer_helper_ = nullptr;
-}
-
 }  // namespace media

media/gpu/android/pooled_shared_image_video_provider.h

@@ -57,8 +57,7 @@ class MEDIA_GPU_EXPORT PooledSharedImageVideoProvider
       base::SequenceBound<GpuHelper> gpu_helper,
       std::unique_ptr<SharedImageVideoProvider> provider);
 
-  class GpuHelperImpl : public GpuHelper,
-                        public gpu::CommandBufferStub::DestructionObserver {
+  class GpuHelperImpl : public GpuHelper {
    public:
     GpuHelperImpl(GetStubCB get_stub_cb);
     ~GpuHelperImpl() override;
@@ -68,9 +67,6 @@ class MEDIA_GPU_EXPORT PooledSharedImageVideoProvider
                          scoped_refptr<CodecImageHolder> codec_image_holder,
                          base::OnceClosure cb) override;
 
-    // gpu::CommandBufferStub::DestructionObserver
-    void OnWillDestroyStub(bool have_context) override;
-
    private:
     void OnSyncTokenCleared(scoped_refptr<CodecImageHolder> codec_image_holder,
                             base::OnceClosure cb);