This should be faster, and also matches the C++ style rules better: static
variables should not have complex class types.

BUG=angleproject:956
TEST=angle_end2end_tests, WebGL conformance tests

Change-Id: I628db95febab309e241be216e2998305525da76e
Reviewed-on: https://chromium-review.googlesource.com/262421Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>