Add a snapshot of flutter/engine/src/build to our sdk

build/secondary/tools/grit/BUILD.gn

Index: build/secondary/tools/grit/BUILD.gn
diff --git a/build/secondary/tools/grit/BUILD.gn b/build/secondary/tools/grit/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..660bf1b1ebcfb553b690052ecf0468ec223499b2
--- /dev/null
+++ b/build/secondary/tools/grit/BUILD.gn
@@ -0,0 +1,27 @@
+# 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.
+
+# This target creates a stamp file that depends on all the sources in the grit
+# directory. By depending on this, a target can force itself to be rebuilt if
+# grit itself changes.
+action("grit_sources") {
+  depfile = "$target_out_dir/grit_sources.d"
+  script = "//build/secondary/tools/grit/stamp_grit_sources.py"
+
+  inputs = [
+    "grit.py",
+  ]
+
+  # Note that we can't call this "grit_sources.stamp" because that file is
+  # implicitly created by GN for script actions.
+  outputs = [
+    "$target_out_dir/grit_sources.script.stamp",
+  ]
+
+  args = [
+    rebase_path("//tools/grit", root_build_dir),
+    rebase_path(outputs[0], root_build_dir),
+    rebase_path(depfile, root_build_dir),
+  ]
+}