Implement WEBGL_compressed_texture_s3tc_srgb

Enables support on non-ANGLE desktop platforms (Mac/Linux) and GLES platforms supporting NV_sRGB_formats (Tegra).

This is tested by WebGL conformance test
conformance/extensions/webgl-compressed-texture-s3tc-srgb.html

BUG=630498
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
TBR=haraken@chromium.org

Review-Url: https://codereview.chromium.org/2337833002
Cr-Commit-Position: refs/heads/master@{#419333}
16 files changed