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/fake_p2p_manager_configuration.h b/fake_p2p_manager_configuration.h
index 206fc57..6272689 100644
--- a/fake_p2p_manager_configuration.h
+++ b/fake_p2p_manager_configuration.h
@@ -39,18 +39,18 @@
   }
 
   // P2PManager::Configuration override
-  virtual base::FilePath GetP2PDir() {
+  base::FilePath GetP2PDir() override {
     return base::FilePath(p2p_dir_);
   }
 
   // P2PManager::Configuration override
-  virtual std::vector<std::string> GetInitctlArgs(bool is_start) {
+  std::vector<std::string> GetInitctlArgs(bool is_start) override {
     return is_start ? initctl_start_args_ : initctl_stop_args_;
   }
 
   // P2PManager::Configuration override
-  virtual std::vector<std::string> GetP2PClientArgs(const std::string &file_id,
-                                                    size_t minimum_size) {
+  std::vector<std::string> GetP2PClientArgs(const std::string &file_id,
+                                            size_t minimum_size) override {
     std::string formatted_command_line = p2p_client_cmdline_format_;
     // Replace {variable} on the passed string.
     ReplaceSubstringsAfterOffset(