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/install_plan.h b/install_plan.h
index 8ccabc6..c592889 100644
--- a/install_plan.h
+++ b/install_plan.h
@@ -98,7 +98,7 @@
   explicit InstallPlanAction(const InstallPlan& install_plan):
     install_plan_(install_plan) {}
 
-  virtual void PerformAction() {
+  void PerformAction() override {
     if (HasOutputPipe()) {
       SetOutputObject(install_plan_);
     }
@@ -108,7 +108,7 @@
   InstallPlan* install_plan() { return &install_plan_; }
 
   static std::string StaticType() { return "InstallPlanAction"; }
-  virtual std::string Type() const { return StaticType(); }
+  std::string Type() const override { return StaticType(); }
 
   typedef ActionTraits<InstallPlanAction>::InputObjectType InputObjectType;
   typedef ActionTraits<InstallPlanAction>::OutputObjectType OutputObjectType;