remove default Uniforms base Ptr

This tripped us up at least once recently.

Change-Id: I902759929e359ce639557bb766b0c6593badc07b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/352093
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
3 files changed