Also adds a simple unit test checking the pruning

BUG=angleproject:937
BUG=395048

Change-Id: I49904c34d1a72949cdc579569967d99c736c7237
Reviewed-on: https://chromium-review.googlesource.com/264415Reviewed-by: Kenneth Russell <kbr@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>