(In reply to Sotaro Ikeda [:sotaro] from comment #0)
Error message was "ASSERTION: Tried to delete invalid animation".
WebRenderBridgeParent::RemoveEpochDataPriorTo() has a problem. Animation was deleted too early in the following STR.
- Request to delete an animation A by WebRenderBridgeParent::RecvDeleteCompositorAnimations()
- Request to re-add the animation A by WebRenderParentCommand::TOpAddCompositorAnimations message
- WebRenderBridgeParent::RemoveEpochDataPriorTo() deletes obsoleted animation
In the STR,  removed animation A even when  re-added animation A again. It should not be deleted.