Command buffer: clear rect for a specific layer/level of the uncleared texture for CopyTexSubImage3D.
BUG=
631934,
636626
CQ_INCLUDE_TRYBOTS=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