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>

buffet/commands/command_queue_unittest.cc

diff --git a/buffet/commands/command_queue_unittest.cc b/buffet/commands/command_queue_unittest.cc
index 117d7bc..2c0e01b 100644
--- a/buffet/commands/command_queue_unittest.cc
+++ b/buffet/commands/command_queue_unittest.cc
@@ -25,7 +25,7 @@
 // Aborts if duplicate commands are added or non-existent commands are removed.
 class FakeDispatchInterface : public buffet::CommandDispachInterface {
  public:
-  virtual void OnCommandAdded(
+  void OnCommandAdded(
       const std::string& command_id,
       const buffet::CommandInstance* command_instance) override {
     CHECK(ids_.insert(command_id).second)
@@ -34,7 +34,7 @@
         << "Command instance already exists";
   }
 
-  virtual void OnCommandRemoved(
+  void OnCommandRemoved(
       const std::string& command_id,
       const buffet::CommandInstance* command_instance) override {
     CHECK_EQ(1, ids_.erase(command_id))