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/update_manager/real_system_provider.h b/update_manager/real_system_provider.h
index d1e20be..c040855 100644
--- a/update_manager/real_system_provider.h
+++ b/update_manager/real_system_provider.h
@@ -24,19 +24,19 @@
   // Initializes the provider and returns whether it succeeded.
   bool Init();
 
-  virtual Variable<bool>* var_is_normal_boot_mode() override {
+  Variable<bool>* var_is_normal_boot_mode() override {
     return var_is_normal_boot_mode_.get();
   }
 
-  virtual Variable<bool>* var_is_official_build() override {
+  Variable<bool>* var_is_official_build() override {
     return var_is_official_build_.get();
   }
 
-  virtual Variable<bool>* var_is_oobe_complete() override {
+  Variable<bool>* var_is_oobe_complete() override {
     return var_is_oobe_complete_.get();
   }
 
-  virtual Variable<bool>* var_is_boot_device_removable() override {
+  Variable<bool>* var_is_boot_device_removable() override {
     return var_is_boot_device_removable_.get();
   }