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Now that the shards are more evenly distributed, this is feasible.
Increase the number of shards to handle the additional ~15 minutes of
run time.

BUG=586033
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_optional_gpu_tests_rel;tryserver.chromium.mac:mac_optional_gpu_tests_rel;tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2133673003
Cr-Commit-Position: refs/heads/master@{#405246}