Add a mechanism for GMs to silently exclude themselves from Gold

This is useful, for example, when trying to test a hardware feature
that isn't supported in the current context.

Bug: skia:8731
Change-Id: I9a363159300c92e4039bfd05400238c27002efb1
Reviewed-on: https://skia-review.googlesource.com/c/189133
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
37 files changed