getExecutionTime requires encoding commands into an otherwise finished command buffer #1331
Description
#870 introduced getExecutionTime as a method on GPUCommandBuffer. On Vulkan and D3D12 it would require the implementation to encode the timestamp commands, but on WebGPU side the GPUCommandEncoder is already closed.
This breaks our command encoding semantics, and I don't think needs to.
Instead, we should fix the getExecutionTime in a way that makes it known at the command encoding time. Also see #1325 (comment)