Report system_version in os version.

Still keep the old behavior if system version is empty for Chrome OS.

Bug: 62426258
Test: check omaha request
Change-Id: Iae111272ec99ebc7d5a132dae7747c72bacd2860
(cherry picked from commit 709dad480f03b13242d953f4ba3f4503629a8e4f)
diff --git a/omaha_request_params.cc b/omaha_request_params.cc
index 3402451..3ba7037 100644
--- a/omaha_request_params.cc
+++ b/omaha_request_params.cc
@@ -77,7 +77,10 @@
   LOG(INFO) << "Running from channel " << image_props_.current_channel;
 
   os_platform_ = constants::kOmahaPlatformName;
-  os_version_ = OmahaRequestParams::kOsVersion;
+  if (!image_props_.system_version.empty())
+    os_version_ = image_props_.system_version;
+  else
+    os_version_ = OmahaRequestParams::kOsVersion;
   if (!in_app_version.empty())
     image_props_.version = in_app_version;