pdf-comparison and image_diff_metric tools

Change-Id: I2ee96581d77c5ded9c9c2dc371a554ebda6b8d26
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/242819
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 299dd65..0e0f9fb 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -2427,6 +2427,13 @@
       ]
     }
   }
+
+  executable("image_diff_metric") {
+    sources = [ "tools/image_diff_metric.cpp" ]
+    deps = [
+      ":skia",
+    ]
+  }
 }
 
 if (is_ios && skia_use_metal) {