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There is only one implementation so there doesn't need to be a virtual
base class. Also remove some unused functions.

Bug: 832243
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I581d47639448efa4a0812f604808a03587269320
Reviewed-on: https://chromium-review.googlesource.com/1119122
Commit-Queue: kylechar <kylechar@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572234}