This covers 'pow-of-small-constant-in-user-defined-function' from
the WebGL CTS. Leave disabled until we have a proper fix.

BUG=angleproject:851

Change-Id: I41a4ad9354e32bb2d48894d75676a9dfe226e9d8
Reviewed-on: https://chromium-review.googlesource.com/269747Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>