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_clock.h b/fake_clock.h
index 54c7c02..c717bdf 100644
--- a/fake_clock.h
+++ b/fake_clock.h
@@ -14,15 +14,15 @@
  public:
   FakeClock() {}
 
-  virtual base::Time GetWallclockTime() {
+  base::Time GetWallclockTime() override {
     return wallclock_time_;
   }
 
-  virtual base::Time GetMonotonicTime() {
+  base::Time GetMonotonicTime() override {
     return monotonic_time_;
   }
 
-  virtual base::Time GetBootTime() {
+  base::Time GetBootTime() override {
     return boot_time_;
   }