Get rid of use of "static const std::string".

BUG=angle:807
TEST=angle_unittests

Change-Id: Ifa4d713deeb25d52a7aafc362a7e4630024fd511
Reviewed-on: https://chromium-review.googlesource.com/225004
Tested-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>