Files services library: Add an "output stream" file impl helper class.

mojo/services/files/public/cpp/BUILD.gn

+# Copyright 2015 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.
+
+import("//build/module_args/mojo.gni")
+import("$mojo_sdk_root/mojo/public/mojo_application.gni")
+import("$mojo_sdk_root/mojo/public/mojo_sdk.gni")
+
+mojo_sdk_source_set("output_stream_file") {
+  # TODO(vtl): This is needed because mojo_sdk.gni doesn't understand relative
+  # dependencies (so without this we can't depend on our interfaces!). Should
+  # this target even be a mojo_sdk_source_set?
+  restrict_external_deps = false
+
+  public_configs = [ "../../../public/build/config:mojo_services" ]
+
+  sources = [
+    "lib/output_stream_file.cc",
+    "output_stream_file.h",
+  ]
+
+  deps = [
+    "../interfaces",
+  ]
+
+  mojo_sdk_deps = [
+    "mojo/public/cpp/bindings",
+    "mojo/public/cpp/environment",
+    "mojo/public/cpp/system",
+  ]
+}
+
+mojo_native_application("apptests") {
+  output_name = "files_cpp_lib_apptests"
+
+  testonly = true
+
+  sources = [
+    "tests/output_stream_file_unittest.cc",
+  ]
+
+  deps = [
+    ":output_stream_file",
+    "../interfaces",
+    "$mojo_sdk_root/mojo/public/cpp/application:standalone",
+    "$mojo_sdk_root/mojo/public/cpp/application:test_support_standalone",
+    "$mojo_sdk_root/mojo/public/cpp/bindings",
+    "$mojo_sdk_root/mojo/public/cpp/environment",
+    "$mojo_sdk_root/mojo/public/cpp/system",
+    "$mojo_sdk_root/mojo/public/cpp/utility",
+    "//testing/gtest",
+  ]
+}