Fix missing uint cases for GLSL and ESSL output. These were causing assertions in several WebGL2 conformance tests. BUG=angleproject:882 BUG=angleproject:1084 BUG=483282 Change-Id: I7651c359a528c95469dcbfb5da2715ae4616b49b Reviewed-on: https://chromium-review.googlesource.com/286592 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Zhenyao Mo <zmo@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>