Refactor fileutils.cc/h and fileutils_unittests.cc into their own targets.

This will allow for custom implementations downstream.

R=kjellander@webrtc.org, phoglund@webrtc.org
BUG=webrtc:6727

Review-Url: https://codereview.webrtc.org/2548713003
Cr-Commit-Position: refs/heads/master@{#15423}
diff --git a/webrtc/test/testsupport/fileutils_unittest.cc b/webrtc/test/testsupport/fileutils_unittest.cc
index e6699af..5e36c35 100644
--- a/webrtc/test/testsupport/fileutils_unittest.cc
+++ b/webrtc/test/testsupport/fileutils_unittest.cc
@@ -37,15 +37,15 @@
  protected:
   FileUtilsTest() {
   }
-  virtual ~FileUtilsTest() {}
+  ~FileUtilsTest() override {}
   // Runs before the first test
   static void SetUpTestCase() {
     original_working_dir_ = webrtc::test::WorkingDir();
   }
-  void SetUp() {
+  void SetUp() override {
     ASSERT_EQ(chdir(original_working_dir_.c_str()), 0);
   }
-  void TearDown() {
+  void TearDown() override {
     ASSERT_EQ(chdir(original_working_dir_.c_str()), 0);
   }
  private: