Basic shell class for CommandPool

Bug b/119827933

Change-Id: Ibbc8ac641b168a15974fd4ff1803b5aff51f48a3
Reviewed-on: https://swiftshader-review.googlesource.com/c/22730Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>