Add tools/imagediff to GN build.

content/shell/BUILD.gn

 # 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.
 
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//tools/grit/grit_rule.gni")
 import("//tools/grit/repack.gni")
 if (is_android) {
   import("//build/config/android/config.gni")
@@ skipping 435 matching lines @@
       configs -= [ "//build/config/win:console" ]
       configs += [ "//build/config/win:windowed" ]
     }
   }
 
   if (is_mac) {
     # TODO(GYP) lots of stuff from GYP file here.
   }
 
   if (!is_android || !is_android_webview_build) {
-    deps += [
-      # NOTE: rely on host build of this target.
-      #'../tools/imagediff/image_diff.gyp:image_diff',  TODO(GYP)
+    # Some tests rely on this tool. It might be nicer if these tests relied on
+    # image diff rather than having content shell depend on it.
+    datadeps = [
+      "//tools/imagediff($host_toolchain)",
     ]
   }
 }
 
 } else {
   # Content shell not ready, make a dummpy to make dependency management easier.
   group("content_shell") {
   }
 }