update_engine: Add override when possible.

Google Style Guide requires to include the "override" keyword
when overriding a method on a derived class, so the compiler will
catch errors if the method is not overriding a member of the base
class.

This patch introduces the "override" keyword when possible.

BUG=None
TEST=FEATURES=test emerge-link update_engine

Change-Id: Ie83d115c5730f3b35b3d95859a54bc1a48e0be7b
Reviewed-on: https://chromium-review.googlesource.com/228928
Tested-by: Alex Deymo <deymo@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>
diff --git a/omaha_request_params_unittest.cc b/omaha_request_params_unittest.cc
index 6458d3a..82d74f1 100644
--- a/omaha_request_params_unittest.cc
+++ b/omaha_request_params_unittest.cc
@@ -33,7 +33,7 @@
   bool DoTest(OmahaRequestParams* out, const string& app_version,
               const string& omaha_url);
 
-  virtual void SetUp() {
+  void SetUp() override {
     // Create a uniquely named test directory.
     ASSERT_TRUE(utils::MakeTempDirectory(kTestDirTemplate,
                                          &test_dir_));
@@ -49,7 +49,7 @@
     params_.SetLockDown(false);
   }
 
-  virtual void TearDown() {
+  void TearDown() override {
     EXPECT_EQ(0, System(string("rm -rf ") + test_dir_));
   }