This moves the debug callback code into a common location. For ease of
access it's in shader_utils since that file has access to the GL API.

Bug: angleproject:5040
Change-Id: Iab9de47c2d520a5618ea6825852f8afa63565c8a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2408715
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>