This reduces the number of values that are added with
the same key.

Bug: angleproject:5853
Change-Id: Ie13408cc7ad419dec06746dc38b753f76292fae6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2824433
Commit-Queue: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>