update_engine: Make the ChromeOS/AOSP Omaha Client transmit sessionid.

As per Omaha's protocol specification, ChromeOS/AOSP
needs to transmit the 'sessionid` attribute in the request.
The format of the 'sessionid' attribute is sent as GUID version 4.

The sessionid is kept throughout the entirety of the update flow.
1. When the <updatecheck> (pings/download/updates) is done, the pings
   to Omaha will send empty sessionids.
2. If there is a schedule error/issue and a new update is scheduled, a
   new sessionid will be applied.
3. During errors/issues, the same sessionid will be used.
4. All new <updatechecks> will start with a fresh sessionid.

BUG=chromium:940515
TEST=cros_workon_make --board=octopus update_engine --test
TEST=/usr/bin/update_engine_client --check_for_update # after bouncing update-engine + check /var/log/update_engine.log. 'sessionid'
attribute will be in the omaha request.

Change-Id: If4d29b630e3ab1b547606ef1c5fb06cc7a9cd61f
Reviewed-on: https://chromium-review.googlesource.com/1658422
Tested-by: Jae Hoon Kim <kimjae@chromium.org>
Commit-Ready: Jae Hoon Kim <kimjae@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/omaha_request_builder_xml.h b/omaha_request_builder_xml.h
index c390b9e..0ba44b8 100644
--- a/omaha_request_builder_xml.h
+++ b/omaha_request_builder_xml.h
@@ -121,7 +121,8 @@
                          int ping_active_days,
                          int ping_roll_call_days,
                          int install_date_in_days,
-                         PrefsInterface* prefs)
+                         PrefsInterface* prefs,
+                         const std::string& session_id)
       : event_(event),
         params_(params),
         ping_only_(ping_only),
@@ -129,7 +130,8 @@
         ping_active_days_(ping_active_days),
         ping_roll_call_days_(ping_roll_call_days),
         install_date_in_days_(install_date_in_days),
-        prefs_(prefs) {}
+        prefs_(prefs),
+        session_id_(session_id) {}
 
   ~OmahaRequestBuilderXml() override = default;
 
@@ -173,6 +175,7 @@
   int ping_roll_call_days_;
   int install_date_in_days_;
   PrefsInterface* prefs_;
+  std::string session_id_;
 
   DISALLOW_COPY_AND_ASSIGN(OmahaRequestBuilderXml);
 };