Implement glGetUniform calls to fix crashes in UniformTest test group.

Tests: UniformTest test group
Bug: angleproject:4817
Change-Id: I132c61e053665b0642667aad51f1f3220d101d14
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2339542
Commit-Queue: Manh Nguyen <nguyenmh@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>