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

Add more DCHECKs.
Same as r5607 except that Thread.StopSoon is fixed.
Review URL: http://codereview.chromium.org/11534

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@5971 0039d316-1c4b-4281-b951-d872f2087c98

base/thread.cc

@@ -131,9 +131,6 @@ void Thread::StopSoon() {
   DCHECK(message_loop_);
 
   message_loop_->PostTask(FROM_HERE, new ThreadQuitTask());
-
-  // The thread can't receive messages anymore.
-  message_loop_ = NULL;
 }
 
 void Thread::ThreadMain() {

base/thread_unittest.cc

@@ -97,8 +97,8 @@ TEST_F(ThreadTest, StopSoon) {
   EXPECT_TRUE(a.Start());
   EXPECT_TRUE(a.message_loop());
   a.StopSoon();
-  EXPECT_FALSE(a.message_loop());
   a.StopSoon();
+  a.Stop();
   EXPECT_FALSE(a.message_loop());
 }