We already update the GL_RENDERER string. We should do the same for
GL_VENDOR so this information is passed along correctly.

Bug: chromium:1152212
Change-Id: I639700bfebdc9e77ec3c6b41fcd6516e8c32139d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2560824Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>