Add fuzz executable to GN

BUILD.gn

 # Copyright 2016 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("gn/shared_sources.gni")
 
 if (!defined(is_skia_standalone)) {
   is_skia_standalone = false
 }
@@ skipping 969 matching lines @@
       ":flags",
       ":gpu_tool_utils",
       ":skia",
       ":tool_utils",
       "//third_party/jsoncpp",
       "//third_party/libmicrohttpd",
       "//third_party/libpng",
     ]
     testonly = true
   }
+
+  executable("fuzz") {
+    sources = [
+      "fuzz/FilterFuzz.cpp",
+      "fuzz/FuzzGradients.cpp",
+      "fuzz/FuzzParsePath.cpp",
+      "fuzz/FuzzPathop.cpp",
+      "fuzz/FuzzScaleToSides.cpp",
+      "fuzz/fuzz.cpp",
+    ]
+    deps = [
+      ":flags",
+      ":skia",
+    ]
+    testonly = true
+  }
 }