The extension entry point can forward to the ES 3.0 entry point. Needed
for capturing the mutiple draw buffers sample.

Bug: angleproject:4433
Change-Id: I1081e6a5ad57eca6dc328947bb0f9037d47fe9f0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2079155Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>