Add a default constructor to sh::ShaderVariable.

This cleans up a few repeated lines of code in shadervars.h.

BUG=angle:466

Change-Id: Ie77130e8420d28df790bf9bc036d52fd053b3ce9
Reviewed-on: https://chromium-review.googlesource.com/206827
Reviewed-by: Nicolas Capens <capn@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>