Allow the GL backend to call BlitFramebuffer through these
extensions.

Bug: angleproject:5474
Change-Id: Ic19b0535056c3e2b098dedc60e2eb8da5f862433
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2602702
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>