Split from https://chromium-review.googlesource.com/c/angle/angle/+/1705035/27

This patch adds entrypoints glDrawArraysInstancedBaseInstanceANGLE,
glDrawElementsInstancedBaseVertexBaseInstanceANGLE,
glMultiDrawArraysInstancedBaseInstanceANGLE,
and glMultiDrawElementsInstancedBaseVertexBaseInstanceANGLE

Implementation will come in a later separate patch.

Bug: chromium:891861, angleproject:3402
Change-Id: I18e19b850cddd79be4798b9ae7efe0680a050c7f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1750125
Commit-Queue: Shrek Shao <shrekshao@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>