Change WebGL API arg nullable behavior.

This is due to a spec change.

BUG=662520
TEST=webgl_conformance,webgl2_conformance
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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

Review-Url: https://codereview.chromium.org/2485443002
Cr-Commit-Position: refs/heads/master@{#430154}
5 files changed