Decouple gl_BaseVertex/gl_BaseInstance uniforms These are builtin uniforms removed in https://github.com/KhronosGroup/WebGL/pull/3278 Decouple them from the original ANGLE_base_vertex_base_instance extension. Make a new ANGLE_base_vertex_base_instance_shader_builtin extension for these builtin uniforms. Bug: angleproject:3402 Change-Id: I77b93917976ce435db9c578c0ade37bff18a42b0 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3290304 Commit-Queue: Shrek Shao <shrekshao@google.com> Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Kenneth Russell <kbr@chromium.org>