Add a compiler option to prune unused function and prototypes Also adds a simple unit test checking the pruning BUG=angleproject:937 BUG=395048 Change-Id: I49904c34d1a72949cdc579569967d99c736c7237 Reviewed-on: https://chromium-review.googlesource.com/264415 Reviewed-by: Kenneth Russell <kbr@chromium.org> Tested-by: Corentin Wallez <cwallez@chromium.org>