Adds support for translating gl_BaseVertex and gl_BaseInstance for implementation of
GL_ANGLE_base_vertex_base_instance.

They will only be available in WebGL 2. Since there's no gl_VertexID and
gl_InstanceID in WebGL 1. It won't be very useful to add them to WebGL
1.

Mostly follow pattern of gl_DrawID of GL_multi_draw

BUG=angleproject:3402,chromium:891861

Change-Id: Ifcd990c52d12f6814127b904e61a779b8d382e0c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1666361
Commit-Queue: Shrek Shao <shrekshao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>