This fixes an omission that out parameter tracking had inherited from
EmulatePrecision. Accurate tracking of when values are written is
required for converting dynamic indexing of vectors and matrices to
function calls.

A new test covering this is added to angle_unittests.

TEST=angle_unittests
BUG=angleproject:1116

Change-Id: I05c5fd60355117d0053b84110748ae221375a790
Reviewed-on: https://chromium-review.googlesource.com/290562Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>