Add linux optional tests to content/test/gpu presubmit.
Add linux optional tests to content/test/gpu presubmit.

This will ensure these tests run on WebGL rolls.

BUG=583702
R=zmo@chromium.org
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/1872503002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#385824}

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