update_engine: fixed warnings from cpplint

Fixed all the cpplint warnings in update engine.

BUG=None
TEST=Unit tests still pass.

Change-Id: I285ae858eec8abe0b26ff203b99a42a200ceb71c
Reviewed-on: https://chromium-review.googlesource.com/204027
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
diff --git a/mock_connection_manager.h b/mock_connection_manager.h
index 71b3019..ffb695a 100644
--- a/mock_connection_manager.h
+++ b/mock_connection_manager.h
@@ -16,7 +16,7 @@
 // logic in update_engine.
 class MockConnectionManager : public ConnectionManager {
  public:
-  MockConnectionManager(SystemState* system_state)
+  explicit MockConnectionManager(SystemState* system_state)
       : ConnectionManager(system_state) {}
 
   MOCK_CONST_METHOD3(GetConnectionProperties,