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/bzip_extent_writer_unittest.cc b/bzip_extent_writer_unittest.cc
index 0aa8709..4cf971b 100644
--- a/bzip_extent_writer_unittest.cc
+++ b/bzip_extent_writer_unittest.cc
@@ -27,12 +27,12 @@
 
 class BzipExtentWriterTest : public ::testing::Test {
  protected:
-  virtual void SetUp() {
+  void SetUp() override {
     memcpy(path_, kPathTemplate, sizeof(kPathTemplate));
     fd_ = mkstemp(path_);
     ASSERT_GE(fd_, 0);
   }
-  virtual void TearDown() {
+  void TearDown() override {
     close(fd_);
     unlink(path_);
   }