Implement robust GetUniform entry points. BUG=angleproject:1354 Change-Id: I07dccc470c57aadd41cae8fce327e4aca58a0bea Reviewed-on: https://chromium-review.googlesource.com/395113 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>