Add support to update_engine_client for -app_version and -omaha_url.

omaha_request_params.h

Index: omaha_request_params.h
diff --git a/omaha_request_params.h b/omaha_request_params.h
index fb4196ea3577c3a641c7c2de2dd9da63d36f6bf9..16def259192b276869c57153c5ffab5c3d82e494 100644
--- a/omaha_request_params.h
+++ b/omaha_request_params.h
@@ -67,11 +67,13 @@ struct OmahaRequestParams {
 
 class OmahaRequestDeviceParams : public OmahaRequestParams {
  public:
-  explicit OmahaRequestDeviceParams() {}
+  OmahaRequestDeviceParams() {}
 
-  // Initializes all the data in the object. Returns true on success,
-  // false otherwise.
-  bool Init();
+  // Initializes all the data in the object. Non-empty
+  // |in_app_version| or |in_update_url| prevents automatic detection
+  // of the parameter. Returns true on success, false otherwise.
+  bool Init(const std::string& in_app_version,
+            const std::string& in_update_url);
 
   // For unit-tests.
   void set_root(const std::string& root) { root_ = root; }