Expose KHR_parallel_shader_compile to WebGL

This exposes KHR_parallel_shader_compile extension to WebGL. With this
extension, applications may benefit from background shader compilation
and program linking, avoiding main thread long blocking for complex
shaders.

BUG=849576

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: I6b62142d28dc14f85d50a820e6a87409d5a371c8
Reviewed-on: https://chromium-review.googlesource.com/1161747
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Jie A Chen <jie.a.chen@intel.com>
Cr-Commit-Position: refs/heads/master@{#591321}
16 files changed