This shrinks the size of the mask for use in cache key classes.

Bug: angleproject:4881
Change-Id: I87c234832c61e6a663c609b7f6da5d69977b21c7
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2399182Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>