Standardize usage of virtual/override/final specifiers.

Standardize usage of virtual/override/final specifiers.

The Google C++ style guide states:

  Explicitly annotate overrides of virtual functions or virtual
  destructors with an override or (less frequently) final specifier.
  Older (pre-C++11) code will use the virtual keyword as an inferior
  alternative annotation. For clarity, use exactly one of override,
  final, or virtual when declaring an override.

To better conform to these guidelines, the following constructs have
been rewritten:

- if a base class has a virtual destructor, then:
    virtual ~Foo();                   ->  ~Foo() override;
- virtual void Foo() override;        ->  void Foo() override;
- virtual void Foo() override final;  ->  void Foo() final;

This patch was automatically generated. The clang plugin can generate
fixit hints, which are suggested edits when it is 100% sure it knows how
to fix a problem. The hints from the clang plugin were applied to the
source tree using the tool in https://codereview.chromium.org/598073004.

BUG=417463
R=asvitkine@chromium.org

Committed: https://crrev.com/4c11274720f68e57412a4bffcee33d942bad0eb8
Cr-Commit-Position: refs/heads/master@{#301491}

[Alexei Svitkine (slow), 2014-10-27 21:01:36]

LGTM

[dcheng, 2014-10-27 21:02:55]

The CQ bit was checked by dcheng@chromium.org

[commit-bot: I haz the power, 2014-10-27 21:04:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/679243002/1

[commit-bot: I haz the power, 2014-10-28 00:01:30]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-28 00:02:12]

Patchset 1 (id:??) landed as
https://crrev.com/4c11274720f68e57412a4bffcee33d942bad0eb8
Cr-Commit-Position: refs/heads/master@{#301491}