This mixin is being removed from the Chromium version of the mixins.
Therefore we have to move it into the additional mixins option of the
generator.

Bug: angleproject:6037
Change-Id: I605b7cbf35d6282754bf5fb26246e32b2edcea10
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2939466Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>