platform2: sweep the lint errors identified by the updated linter

cpplint.py has been updated and identified new issues in existing
code. Stuff like overridden functions that specify 'override' should
not be marked as 'virtual', and constructors with no parameters
should not be marked as 'explicit'.

BUG=None
TEST=cpplint.py `find ./platform2 -name *.cc -or -name *.h`

Change-Id: Ibb9de43286d874d076ffd5ebb1b13c36ec794f01
Reviewed-on: https://chromium-review.googlesource.com/211950
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
diff --git a/buffet/any_internal_impl.h b/buffet/any_internal_impl.h
index cbf0bb8..5d93200 100644
--- a/buffet/any_internal_impl.h
+++ b/buffet/any_internal_impl.h
@@ -60,12 +60,12 @@
 struct TypedData : public Data {
   explicit TypedData(const T& value) : value_(value) {}
 
-  virtual const std::type_info& GetType() const override { return typeid(T); }
-  virtual void CopyTo(Buffer* buffer) const override;
-  virtual bool IsConvertibleToInteger() const override {
+  const std::type_info& GetType() const override { return typeid(T); }
+  void CopyTo(Buffer* buffer) const override;
+  bool IsConvertibleToInteger() const override {
     return std::is_integral<T>::value || std::is_enum<T>::value;
   }
-  virtual intmax_t GetAsInteger() const override {
+  intmax_t GetAsInteger() const override {
     intmax_t int_val = 0;
     bool converted = TryConvert(value_, &int_val);
     CHECK(converted) << "Unable to convert value of type " << typeid(T).name()