Add test for gl_VertexID and DrawElements This fails on the WebGL CTS conformance2/rendering/vertex-id.html. Repoing here to make it easier to debug/find/fix. Bug: angleproject:6688 Change-Id: I0a4bafc5700c77e0b6c44b067167fc4a60e63c22 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3276708 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Kenneth Russell <kbr@chromium.org> Commit-Queue: Gregg Tavares <gman@chromium.org>