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Translators may be null (e.g. for passthrough, or in case of failure), so only
AddRef them if they are valid.

Bug: 779954
Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I8f0039eaf2bdaf76a58e74317af68c13d252bdd5
Reviewed-on: https://chromium-review.googlesource.com/746993Reviewed-by: Victor Miura <vmiura@chromium.org>
Commit-Queue: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512894}