ui/gl: Ensure GLSL shader version is consistent with GL version override

When downgrading the GL version exposed to skia, we only override the GL
version string. Also override the GLSL version string to ensure it is
consistent with exposed GL version.

R=piman@chromium.org

Bug: 978354
Change-Id: I7863e627ce32863f796dd708f866aaab9d321a4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1703267
Commit-Queue: Khushal <khushalsagar@chromium.org>
Auto-Submit: Khushal <khushalsagar@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678432}