Reland r68893 after fixing allowing blocking IO on shutdown.

chrome/browser/printing/printer_query.cc

 // Copyright (c) 2010 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/printing/printer_query.h"
 
 #include "base/message_loop.h"
+#include "base/thread_restrictions.h"
 #include "chrome/browser/printing/print_job_worker.h"
 
 namespace printing {
 
 PrinterQuery::PrinterQuery()
     : io_message_loop_(MessageLoop::current()),
       ALLOW_THIS_IN_INITIALIZER_LIST(worker_(new PrintJobWorker(this))),
       is_print_dialog_box_shown_(false),
       cookie_(PrintSettings::NewCookie()),
       last_status_(PrintingContext::FAILED) {
@@ skipping 71 matching lines @@
       &PrintJobWorker::GetSettings,
       is_print_dialog_box_shown_,
       parent_view,
       expected_page_count,
       has_selection,
       use_overlays));
 }
 
 void PrinterQuery::StopWorker() {
   if (worker_.get()) {
+    // http://crbug.com/66082: We're blocking on the PrinterQuery's worker
+    // thread.  It's not clear to me if this may result in blocking the current
+    // thread for an unacceptable time.  We should probably fix it.
+    base::ThreadRestrictions::ScopedAllowIO allow_io;
     worker_->Stop();
     worker_.reset();
   }
 }
 
 bool PrinterQuery::is_print_dialog_box_shown() const {
   return is_print_dialog_box_shown_;
 }
 
 bool PrinterQuery::is_callback_pending() const {
   return callback_.get() != NULL;
 }
 
 bool PrinterQuery::is_valid() const {
   return worker_.get() != NULL;
 }
 
 }  // namespace printing