Mark WEBGL_compressed_texture_s3tc_srgb as draft

Plus add TODO for https://bugs.chromium.org/p/chromium/issues/detail?id=647892

BUG=630498
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.win:win_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2346973004
Cr-Commit-Position: refs/heads/master@{#419583}
2 files changed