Output unknown enum to aid debugging When investigating issues with apps it's handy to see the actual enum value, not just that it's an invalid value. Based on http://crrev.com/c/2182578 by Courtney Goeltzenleuchter. Bug: b/155487768 Change-Id: I91611cb620a3bf2d302d667e0f168ca5cb6c9d3c Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2507637 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com> Reviewed-by: Tim Van Patten <timvp@google.com>