Create gles31 context for webgl2-compute in passthrough decoder

With this change, all webgl/webgl2 APIs can be run on webgl2-compute
context for passthrough command buffer.

Bug: 859249
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: Ia3c5febb7f5cbeb16d560ee05362c4746f97d5d6
Reviewed-on: https://chromium-review.googlesource.com/1141434
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Jiajia Qin <jiajia.qin@intel.com>
Cr-Commit-Position: refs/heads/master@{#576293}
10 files changed