Upgrade gl2ext.h.
Upgrade gl2ext.h.

Includes a new entry point needed for disjoint timer queries. This
header is copied exactly from Khronos's official repo. We add one
change so that we can include special ANGLE modifications in
gl2ext_angle.h.

Bug: angleproject:4433
Change-Id: I6c9cbdfa73ca3de052ff49c5ea91eb5e9a9ee699
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2080596
Commit-Queue: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle