Reland "add a stub for running fiddles as GMs"

This is a reland of adfc22df860a9175713d50a507bf5dbb3ee32415

PS2+3 restrict this to just local builds: no G3, no Android, etc.

Original change's description:
> add a stub for running fiddles as GMs
>
> This is about the 15th time I've hacked this up.
>
> Change-Id: I0c75c0d04133d867e942c39eba693fd7227ba0f5
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/233240
> Reviewed-by: Brian Salomon <bsalomon@google.com>
> Commit-Queue: Mike Klein <mtklein@google.com>

Change-Id: I7b8e7418edc4949c2f3e07ddf01f0643344397d1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/233242
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 0a87f3c..c810691 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1719,6 +1719,10 @@
       "modules/sksg",
       "modules/skshaper",
     ]
+    if (is_skia_dev_build) {
+      sources += [ "gm/fiddle.cpp" ]
+      deps += [ ":skia.h" ]
+    }
     public_deps = [
       ":gpu_tool_utils",
     ]