Bring back the harness from SkSLGLSLTest as a testbed for debugging.

The code here is resuscitated from the GLSL testing harness at
http://review.skia.org/317771 . Testing and debugging in dm is simpler
than debugging skslc when we encounter compiler issues.

Change-Id: I492dd0bbd2f0ee0b3b6a1b5253da54d72dc1eb3b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/319032
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
2 files changed