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 <jmadill@chromium.org> Reviewed-by: Zhenyao Mo <zmo@chromium.org> Tested-by: Zhenyao Mo <zmo@chromium.org>