Make sure abs-path gn labels have separate target_gen_dirs

tools/gn/function_get_path_info_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 "testing/gtest/include/gtest/gtest.h"
 #include "tools/gn/functions.h"
 #include "tools/gn/test_with_scope.h"
 
 namespace {
 
@@ skipping 92 matching lines @@
 #endif
 }
 
 // Note build dir is "//out/Debug/".
 TEST_F(GetPathInfoTest, GenDir) {
   EXPECT_EQ("//out/Debug/gen/src/foo/foo", Call("foo/bar.txt", "gen_dir"));
   EXPECT_EQ("//out/Debug/gen/src/foo/bar", Call("bar/", "gen_dir"));
   EXPECT_EQ("//out/Debug/gen/src/foo", Call(".", "gen_dir"));
   EXPECT_EQ("//out/Debug/gen/src/foo", Call("bar", "gen_dir"));
   EXPECT_EQ("//out/Debug/gen/foo", Call("//foo/bar.txt", "gen_dir"));
-  // System paths go into the root obj directory.
-  EXPECT_EQ("//out/Debug/gen", Call("/foo/bar.txt", "gen_dir"));
+  // System paths go into the ABS_PATH gen directory
+  EXPECT_EQ("//out/Debug/gen/ABS_PATH/foo", Call("/foo/bar.txt", "gen_dir"));
+#if defined(OS_WIN)
+  EXPECT_EQ("//out/Debug/gen/ABS_PATH/C/foo",
+            Call("/C:/foo/bar.txt", "out_dir"));
+#endif
 }