This is needed to effectively use the new APIs to get shader variable info.

BUG=angle::770
TEST=ShaderVariableTest

Change-Id: Ia591eb567868ebe898f4a7449c64167ad212f59b
Reviewed-on: https://chromium-review.googlesource.com/221388Tested-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>