Add row-sharing packing tests for VariablePacker_test.cpp
Add row-sharing packing tests for VariablePacker_test.cpp

Recent test failures leads to suspicion that our variable packing algorithm is
buggy - turns out to be an underlying driver bug.

With the added test cases, such suspicion shouldn't even arise.

BUG=angleproject:1142
TEST=angle_unittests

Change-Id: I1fb3c5c7798d9ad17668a3d633286e031da79cab
Reviewed-on: https://chromium-review.googlesource.com/295901
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Zhenyao Mo <655bdf1d95ff956cd68886dd9e86ee5e987c9dbb@chromium.org>
Tested-by: Zhenyao Mo <655bdf1d95ff956cd68886dd9e86ee5e987c9dbb@chromium.org>

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