Add image_diff from the chromium repo.

sky/tools/imagediff/BUILD.gn

Index: sky/tools/imagediff/BUILD.gn
diff --git a/sky/tools/imagediff/BUILD.gn b/sky/tools/imagediff/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..887f4123f333eda2350f7b4158f050ce2ab5d00d
--- /dev/null
+++ b/sky/tools/imagediff/BUILD.gn
@@ -0,0 +1,20 @@
+# Copyright 2014 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+executable("imagediff") {
+  output_name = "image_diff"  # Different than dir nam for historical reasons.
+  sources = [
+    "image_diff.cc",
+    "image_diff_png.h",
+    "image_diff_png.cc",
+  ]
+
+  configs += [ "//build/config/compiler:wexit_time_destructors" ]
+
+  deps = [
+    "//base",
+    "//third_party/libpng",
+    "//third_party/zlib",
+  ]
+}