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>

BUILD.gn

@@ -1713,6 +1713,7 @@ if (skia_enable_tools) {
       ":etc1",
       ":flags",
       ":skia",
+      ":skia.h",
       ":tool_utils",
       "modules/skottie",
       "modules/skottie:gm",

gm/fiddle.cpp

@@ -0,0 +1,15 @@
+/*
+ * Copyright 2019 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "gm/gm.h"
+#include "skia.h"
+
+static void draw(SkCanvas* canvas);
+DEF_SIMPLE_GM(fiddle, canvas, 256, 256) { draw(canvas); }
+
+// Paste your fiddle.skia.org code over this stub.
+void draw(SkCanvas*) {}

gn/gm.gni

@@ -152,6 +152,7 @@ gm_sources = [
   "$_gm/extractbitmap.cpp",
   "$_gm/fadefilter.cpp",
   "$_gm/fatpathfill.cpp",
+  "$_gm/fiddle.cpp",
   "$_gm/filltypes.cpp",
   "$_gm/filltypespersp.cpp",
   "$_gm/filterbitmap.cpp",