Move skdiff tool and add to gn build.

I really wanted this today, so I got it working again.

Change-Id: I1a37d48d4806198b55c59d1df5ff15a03500195f
Reviewed-on: https://skia-review.googlesource.com/3383
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 7935e74..e1f0fc8 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1061,4 +1061,18 @@
     ]
     testonly = true
   }
+
+  executable("skdiff") {
+    sources = [
+      "tools/skdiff/skdiff.cpp",
+      "tools/skdiff/skdiff_html.cpp",
+      "tools/skdiff/skdiff_main.cpp",
+      "tools/skdiff/skdiff_utils.cpp",
+    ]
+    deps = [
+      ":skia",
+      ":tool_utils",
+    ]
+    testonly = true
+  }
 }