GL: Fix share groups for display texture sharing. Chrome uses display-level texture share groups for WebGL contexts. When this extension is in use, make sure all internal contexts are created in the same share group. Bug: chromium:1268830, chromium:1268389, chromium:1268218 Change-Id: I3925dffb9bb23fbbc7adcd3bee8cd62323b993e5 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3285213 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Commit-Queue: Geoff Lang <geofflang@chromium.org>