Expand empty directories in GN to "."

tools/gn/substitution_writer_unittest.cc

 // 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.
 
 #include <sstream>
 
 #include "testing/gtest/include/gtest/gtest.h"
 #include "tools/gn/err.h"
 #include "tools/gn/escape.h"
 #include "tools/gn/substitution_list.h"
@@ skipping 172 matching lines @@
   EXPECT_TRUE(SubstitutionWriter::GetTargetSubstitution(
       &target, SUBSTITUTION_LABEL, &result));
   EXPECT_EQ("//foo/bar:baz", result);
 
   EXPECT_TRUE(SubstitutionWriter::GetTargetSubstitution(
       &target, SUBSTITUTION_ROOT_GEN_DIR, &result));
   EXPECT_EQ("gen", result);
 
   EXPECT_TRUE(SubstitutionWriter::GetTargetSubstitution(
       &target, SUBSTITUTION_ROOT_OUT_DIR, &result));
-  EXPECT_EQ("", result);
+  EXPECT_EQ(".", result);
 
   EXPECT_TRUE(SubstitutionWriter::GetTargetSubstitution(
       &target, SUBSTITUTION_TARGET_GEN_DIR, &result));
   EXPECT_EQ("gen/foo/bar", result);
 
   EXPECT_TRUE(SubstitutionWriter::GetTargetSubstitution(
       &target, SUBSTITUTION_TARGET_OUT_DIR, &result));
   EXPECT_EQ("obj/foo/bar", result);
 
   EXPECT_TRUE(SubstitutionWriter::GetTargetSubstitution(
@@ skipping 42 matching lines @@
 
   // Test that we handle paths that end up in the root build dir properly
   // (no leading "./" or "/").
   Err err;
   SubstitutionPattern pattern;
   ASSERT_TRUE(
       pattern.Parse("{{root_out_dir}}/{{target_output_name}}.so", NULL, &err));
 
   OutputFile output = SubstitutionWriter::ApplyPatternToLinkerAsOutputFile(
       &target, tool, pattern);
-  EXPECT_EQ("libbaz.so", output.value());
+  EXPECT_EQ("./libbaz.so", output.value());
 }