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

build/toolchain/goma.gni

Index: build/toolchain/goma.gni
diff --git a/build/toolchain/goma.gni b/build/toolchain/goma.gni
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+++ b/build/toolchain/goma.gni
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+# Copyright (c) 2013 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.
+
+# Defines the configuration of Goma.
+#
+# This is currently designed to match the GYP build exactly, so as not to break
+# people during the transition.
+
+declare_args() {
+  # Set to true to enable distributed compilation using Goma.
+  use_goma = false
+
+  # Set the default value based on the platform.
+  if (is_win) {
+    # Absolute directory containing the Goma source code.
+    goma_dir = "C:\goma\goma-win"
+  } else {
+    # Absolute directory containing the Goma source code.
+    goma_dir = getenv("HOME") + "/goma"
+  }
+}