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Refactors and cleans-up the code related to the GLHelperScalar shader
programs and adds it to GLScaler. Also, unit testing of the shader
programs was non-existent (only higher-level integration testing was
being done), so much of this CL is about adding testing for each of
them to confirm correct behaviors.

Bug: 870036
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel
Change-Id: I7714b7cf6ebef2cd2778ba146728e6d225b1467a
Reviewed-on: https://chromium-review.googlesource.com/1214662
Commit-Queue: Yuri Wiitala <miu@chromium.org>
Reviewed-by: Xiangjun Zhang <xjz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591118}