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/mock_p2p_manager.h b/mock_p2p_manager.h
index a0ed920..6b5a75e 100644
--- a/mock_p2p_manager.h
+++ b/mock_p2p_manager.h
@@ -60,7 +60,7 @@
             &FakeP2PManager::CountSharedFiles));
   }
 
-  virtual ~MockP2PManager() {}
+  ~MockP2PManager() override {}
 
   // P2PManager overrides.
   MOCK_METHOD1(SetDevicePolicy, void(const policy::DevicePolicy*));