The VariablePacker does not check the staticUse flag, variables should
be pre-filtered according to their staticUse flag before passing them
to CheckVariablesInPackingLimits if that's desired.

The names of the variables are also not relevant to the packing. We
keep the "name" field to make the code easier to debug, but updating
the mappedName is not useful.

This will make implementing arrays of arrays simpler.

BUG=angleproject:2125
TEST=angle_unittests

Change-Id: I5ce91885f6478ad436e6fa60ca9675e161d10256
Reviewed-on: https://chromium-review.googlesource.com/730104Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>