Add a mechanism for command buffer to conditionally allow ES3 enums.

      Although ultimately we want to remove validators from command buffer, but the
fastest way to allow an experimental WebGL 2 is actually appending the current
validators.

Appended the BufferTarget as an sample to make sure code generator works.

BUG=463599
TEST=gpu_unittests, webgl conformance tests
R=sievers@chromium.org

Review URL: https://codereview.chromium.org/987123003

Cr-Commit-Position: refs/heads/master@{#319819}