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/update_manager/generic_variables_unittest.cc b/update_manager/generic_variables_unittest.cc
index 7de6725..65f7f51 100644
--- a/update_manager/generic_variables_unittest.cc
+++ b/update_manager/generic_variables_unittest.cc
@@ -133,7 +133,7 @@
 
 class UmAsyncCopyVariableTest : public ::testing::Test {
  public:
-  void TearDown() {
+  void TearDown() override {
     // No remaining event on the main loop.
     EXPECT_EQ(0, RunGMainLoopMaxIterations(1));
   }