Add 'UniformBlockIndex' struct Add the new structure 'UniformBlockIndex'. This is in preparation for capturing glUniformBlockBinding() calls, which requires the (new) corresponding ParamType::TUniformBlockIndex. Bug: angleproject:5564 Change-Id: I88613dd96d058659dd253e8088fab49670c32316 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2651287 Commit-Queue: Tim Van Patten <timvp@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Cody Northrop <cnorthrop@google.com>