Adding support for GL_CHROMIUM_copy_texture, part 1

Part 1 of 2. This change just adds the gl API entrypoint
so that the chromium/ganesh bindings can set it. Once
the chromium part is landed, we will add full support for
the extension

BUG=crbug.com/415100
R=bsalomon@google.com

Author: junov@chromium.org

Review URL: https://codereview.chromium.org/594663002