This will allow the Trace tests to load a custom GL after the GL
window has already loaded the default entry points.

Bug: angleproject:4845
Change-Id: Ic1b65b6cf1a582ea7c2f58a9f76077760e5cf0d8
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2315626Reviewed-by: Manh Nguyen <nguyenmh@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>