Simplifies implementation by defining a POSIX portability layer; adds the death test style flag to --help.

test/gtest-options_test.cc

@@ -150,22 +150,14 @@ class XmlOutputChangeDirTest : public Test {
  protected:
   virtual void SetUp() {
     original_working_dir_ = FilePath::GetCurrentDir();
-    ChDir("..");
+    posix::chdir("..");
     // This will make the test fail if run from the root directory.
     EXPECT_STRNE(original_working_dir_.c_str(),
                  FilePath::GetCurrentDir().c_str());
   }
 
   virtual void TearDown() {
-    ChDir(original_working_dir_.c_str());
-  }
-
-  void ChDir(const char* dir) {
-#if GTEST_OS_WINDOWS
-    _chdir(dir);
-#else
-    chdir(dir);
-#endif
+    posix::chdir(original_working_dir_.c_str());
   }
 
   FilePath original_working_dir_;