Addition of the entry points for
GL_EXT_primitive_bounding_box extension.

Bug: angleproject:5896
Change-Id: I0dce407d2826ce3f730125d7c03f1233420f6780
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2849615Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>