Don't send machine and user ID to Omaha anymore. Send a/r pings instead.

This avoids sending a unique ID in order to track active user counts.
Note that this CL doesn't remove the machine/user/Omaha ID/file from
the params object -- it just makes them unused/obsolete. Removal will
be done in a subsequent CL in an effort to make this CL smaller.

BUG=1439
TEST=unit tests, x86-generic, arm-generic, gmerged and inspected logs

Review URL: http://codereview.chromium.org/2856070
diff --git a/update_attempter.h b/update_attempter.h
index cd7f480..6fd4c16 100644
--- a/update_attempter.h
+++ b/update_attempter.h
@@ -38,8 +38,9 @@
 class UpdateAttempter : public ActionProcessorDelegate,
                         public DownloadActionDelegate {
  public:
-  UpdateAttempter(MetricsLibraryInterface* metrics_lib)
+  UpdateAttempter(PrefsInterface* prefs, MetricsLibraryInterface* metrics_lib)
       : dbus_service_(NULL),
+        prefs_(prefs),
         metrics_lib_(metrics_lib),
         status_(UPDATE_STATUS_IDLE),
         download_progress_(0.0),
@@ -120,6 +121,9 @@
   // pointer to the OmahaResponseHandlerAction in the actions_ vector;
   std::tr1::shared_ptr<OmahaResponseHandlerAction> response_handler_action_;
 
+  // Pointer to the preferences store interface.
+  PrefsInterface* prefs_;
+
   // Pointer to the UMA metrics collection library.
   MetricsLibraryInterface* metrics_lib_;