EGL: Add const to several methods. This is in preparatino for auto-generating the EGL validation header. The auto-generation script will force multiple parameters to const. Bug: angleproject:2621 Change-Id: I04e442c6ff118fd7c296341f12f442901f6fb8c0 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2552979 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Cody Northrop <cnorthrop@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>