Speeds up project generation a lot.

BUG=angleproject:981

Change-Id: I3cb340baa8da3a30a2c861b584ba0e7758da5b4d
Reviewed-on: https://chromium-review.googlesource.com/266873Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>