exo: Directly create GpuMemoryBufferImpl when a handle is available.

gpu/ipc/client/gpu_memory_buffer_impl.h

Index: gpu/ipc/client/gpu_memory_buffer_impl.h
diff --git a/gpu/ipc/client/gpu_memory_buffer_impl.h b/gpu/ipc/client/gpu_memory_buffer_impl.h
index a8dc56b7e3ad0a3ed0f09efd24cb6fc985be87f8..43e23e34bc553093dca41bda2bf42b8e2dad96dc 100644
--- a/gpu/ipc/client/gpu_memory_buffer_impl.h
+++ b/gpu/ipc/client/gpu_memory_buffer_impl.h
@@ -24,9 +24,9 @@ class GPU_EXPORT GpuMemoryBufferImpl : public gfx::GpuMemoryBuffer {
   ~GpuMemoryBufferImpl() override;
 
   // Creates an instance from the given |handle|. |size| and |internalformat|
-  // should match what was used to allocate the |handle|. |callback| is
-  // called when instance is deleted, which is not necessarily on the same
-  // thread as this function was called on and instance was created on.
+  // should match what was used to allocate the |handle|. |callback|, if
+  // non-null, is called when instance is deleted, which is not necessarily on
+  // the same thread as this function was called on and instance was created on.
   static std::unique_ptr<GpuMemoryBufferImpl> CreateFromHandle(
       const gfx::GpuMemoryBufferHandle& handle,
       const gfx::Size& size,