Add superficial unit test for PrintJob. That's a start....

chrome/browser/printing/print_job.cc

Index: chrome/browser/printing/print_job.cc
===================================================================
--- chrome/browser/printing/print_job.cc	(revision 5751)
+++ chrome/browser/printing/print_job.cc	(working copy)
@@ -24,6 +24,8 @@
       is_job_pending_(false),
       is_print_dialog_box_shown_(false),
       is_canceling_(false) {
+  DCHECK(ui_message_loop_);
+  ui_message_loop_->AddDestructionObserver(this);
 }
 
 PrintJob::PrintJob()
@@ -34,14 +36,18 @@
       is_job_pending_(false),
       is_print_dialog_box_shown_(false),
       is_canceling_(false) {
+  DCHECK(ui_message_loop_);
+  ui_message_loop_->AddDestructionObserver(this);
 }
 
 PrintJob::~PrintJob() {
+  ui_message_loop_->RemoveDestructionObserver(this);
   // The job should be finished (or at least canceled) when it is destroyed.
   DCHECK(!is_job_pending_);
   DCHECK(!is_print_dialog_box_shown_);
   DCHECK(!is_canceling_);
-  DCHECK(worker_->message_loop() == NULL);
+  if (worker_.get())
+    DCHECK(worker_->message_loop() == NULL);
   DCHECK_EQ(ui_message_loop_, MessageLoop::current());
 }
 
@@ -142,6 +148,10 @@
   return document_->cookie();
 }
 
+void PrintJob::WillDestroyCurrentMessageLoop() {
+  NOTREACHED();
+}
+
 void PrintJob::GetSettings(GetSettingsAskParam ask_user_for_settings,
                            HWND parent_window) {
   DCHECK_EQ(ui_message_loop_, MessageLoop::current());