Also adds a simple unit test checking the pruning

BUG=angleproject:937
BUG=395048

Change-Id: I88440378f66178dcebebcd596f8f80235903f20e
Reviewed-on: https://chromium-review.googlesource.com/264568Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>