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/p2p_manager_unittest.cc b/p2p_manager_unittest.cc
index 12b6709..79804f3 100644
--- a/p2p_manager_unittest.cc
+++ b/p2p_manager_unittest.cc
@@ -46,10 +46,10 @@
 class P2PManagerTest : public testing::Test {
  protected:
   P2PManagerTest() : fake_um_(&fake_clock_) {}
-  virtual ~P2PManagerTest() {}
+  ~P2PManagerTest() override {}
 
   // Derived from testing::Test.
-  virtual void SetUp() {
+  void SetUp() override {
     test_conf_ = new FakeP2PManagerConfiguration();
 
     // Allocate and install a mock policy implementation in the fake Update
@@ -63,7 +63,7 @@
                                          "cros_au", 3,
                                          base::TimeDelta::FromDays(5)));
   }
-  virtual void TearDown() {}
+  void TearDown() override {}
 
   // The P2PManager::Configuration instance used for testing.
   FakeP2PManagerConfiguration *test_conf_;