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/file_descriptor.h b/file_descriptor.h
index 3b1f1b2..d86d4ca 100644
--- a/file_descriptor.h
+++ b/file_descriptor.h
@@ -82,19 +82,18 @@
 class EintrSafeFileDescriptor : public FileDescriptor {
  public:
   EintrSafeFileDescriptor() : fd_(-1) {}
-  virtual ~EintrSafeFileDescriptor() {}
 
   // Interface methods.
-  virtual bool Open(const char* path, int flags, mode_t mode);
-  virtual bool Open(const char* path, int flags);
-  virtual ssize_t Read(void* buf, size_t count);
-  virtual ssize_t Write(const void* buf, size_t count);
-  virtual bool Close();
-  virtual void Reset();
-  virtual bool IsSettingErrno() {
+  bool Open(const char* path, int flags, mode_t mode) override;
+  bool Open(const char* path, int flags) override;
+  ssize_t Read(void* buf, size_t count) override;
+  ssize_t Write(const void* buf, size_t count) override;
+  bool Close() override;
+  void Reset() override;
+  bool IsSettingErrno() override {
     return true;
   }
-  virtual bool IsOpen() {
+  bool IsOpen() override {
     return (fd_ >= 0);
   }