Add tools/imagediff to GN build.

tools/imagediff/BUILD.gn

Index: tools/imagediff/BUILD.gn
diff --git a/chrome/plugin/BUILD.gn b/tools/imagediff/BUILD.gn
similarity index 55%
copy from chrome/plugin/BUILD.gn
copy to tools/imagediff/BUILD.gn
index 76e4109793243aa9990a9a8ae88c48e3b3a1fb2c..887f4123f333eda2350f7b4158f050ce2ab5d00d 100644
--- a/chrome/plugin/BUILD.gn
+++ b/tools/imagediff/BUILD.gn
@@ -2,17 +2,19 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-static_library("plugin") {
+executable("imagediff") {
+  output_name = "image_diff"  # Different than dir nam for historical reasons.
   sources = [
-    "chrome_content_plugin_client.cc",
-    "chrome_content_plugin_client.h",
+    "image_diff.cc",
+    "image_diff_png.h",
+    "image_diff_png.cc",
   ]
 
   configs += [ "//build/config/compiler:wexit_time_destructors" ]
 
   deps = [
     "//base",
-    "//chrome:strings",
-    "//content/public/plugin",
+    "//third_party/libpng",
+    "//third_party/zlib",
   ]
 }