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/nl80211_message.h b/nl80211_message.h
index 1b336f2..7c907cc 100644
--- a/nl80211_message.h
+++ b/nl80211_message.h
@@ -489,7 +489,7 @@
   friend struct
       base::DefaultLazyInstanceTraits<Nl80211MessageDataCollector>;
 
-  explicit Nl80211MessageDataCollector();
+  Nl80211MessageDataCollector();
 
  private:
   // In order to limit the output from this class, I keep track of types I