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_attempter.h b/update_attempter.h
index ec4cba2..f29b4e8 100644
--- a/update_attempter.h
+++ b/update_attempter.h
@@ -30,7 +30,7 @@
 struct UpdateEngineService;
 
 namespace policy {
-  class PolicyProvider;
+class PolicyProvider;
 }
 
 namespace chromeos_update_engine {