Add tools/imagediff to GN build.

content/shell/BUILD.gn

Index: content/shell/BUILD.gn
diff --git a/content/shell/BUILD.gn b/content/shell/BUILD.gn
index f6e7f4f3f16dc169f84ebdbc2e0d17a148a480d2..cc60ec72ee8a40ccf8abbea61cc6bc9b6d31f968 100644
--- a/content/shell/BUILD.gn
+++ b/content/shell/BUILD.gn
@@ -453,9 +453,10 @@ executable("content_shell") {
   }
 
   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)",
     ]
   }
 }