Add GN output prefix override and allow empty output extensions.

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 "build/build_config.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "tools/gn/err.h"
 #include "tools/gn/escape.h"
@@ skipping 250 matching lines @@
 
   // Test that we handle paths that end up in the root build dir properly
   // (no leading "./" or "/").
   SubstitutionPattern pattern;
   ASSERT_TRUE(pattern.Parse("{{root_out_dir}}/{{target_output_name}}.so",
                             nullptr, &err));
 
   OutputFile output = SubstitutionWriter::ApplyPatternToLinkerAsOutputFile(
       &target, tool, pattern);
   EXPECT_EQ("./libbaz.so", output.value());
+
+  // Output extensions can be overridden.
+  target.set_output_extension("extension");
+  EXPECT_EQ(".extension",
+            SubstitutionWriter::GetLinkerSubstitution(
+                &target, tool, SUBSTITUTION_OUTPUT_EXTENSION));
+  target.set_output_extension("");
+  EXPECT_EQ("",
+            SubstitutionWriter::GetLinkerSubstitution(
+                &target, tool, SUBSTITUTION_OUTPUT_EXTENSION));
 }