Some drivers emit warnings about enabled extensions not being present
dispite the 'enable' flag only being a request. Supress these warnings
to clean up the Chrome console.

Also request GL_EXT_gpu_shader5 in addition to GL_ARB_gpu_shader5 to
maximize reach.

BUG=chromium:1066212

Change-Id: If48cc60b8533f6ffedb727e845a4ca784d98c07c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2129111Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Geoff Lang <geofflang@chromium.org>