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/omaha_request_params.h b/omaha_request_params.h
index b370bb7..86a18a6 100644
--- a/omaha_request_params.h
+++ b/omaha_request_params.h
@@ -25,11 +25,11 @@
 // essential state needed for the processing of the request/response.  The
 // strings in this struct should not be XML escaped.
 //
-// TODO (jaysri): chromium-os:39752 tracks the need to rename this class to
+// TODO(jaysri): chromium-os:39752 tracks the need to rename this class to
 // reflect its lifetime more appropriately.
 class OmahaRequestParams {
  public:
-  OmahaRequestParams(SystemState* system_state)
+  explicit OmahaRequestParams(SystemState* system_state)
       : system_state_(system_state),
         os_platform_(kOsPlatform),
         os_version_(kOsVersion),