Use ScopedNestedTaskAllower instead of background thread for print dialog

Tasks need to be processed for painting to occur in other browser
windows while the modal dialog is up in Aura.

R=vitalybuka@chromium.org
BUG=342697,180997,343848

Review URL: https://codereview.chromium.org/164303002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251373 0039d316-1c4b-4281-b951-d872f2087c98

printing/printing_context_win.cc

@@ -175,7 +175,6 @@ PrintingContextWin::~PrintingContextWin() {
   ReleaseContext();
 }
 
-// TODO(vitalybuka): Implement as ui::BaseShellDialog crbug.com/180997.
 void PrintingContextWin::AskUserForSettings(
     gfx::NativeView view, int max_pages, bool has_selection,
     const PrintSettingsCallback& callback) {
@@ -238,6 +237,14 @@ void PrintingContextWin::AskUserForSettings(
     dialog_options.Flags |= PD_NOPAGENUMS;
   }
 
+  // Note that this cannot use ui::BaseShellDialog as the print dialog is
+  // system modal: opening it from a background thread can cause Windows to
+  // get the wrong Z-order which will make the print dialog appear behind the
+  // browser frame (but still being modal) so neither the browser frame nor
+  // the print dialog will get any input. See http://crbug.com/342697
+  // http://crbug.com/180997 for details.
+  base::MessageLoop::ScopedNestableTaskAllower allow(
+      base::MessageLoop::current());
   HRESULT hr = (*print_dialog_func_)(&dialog_options);
   if (hr != S_OK) {
     ResetSettings();

printing/printing_context_win_unittest.cc

@@ -10,6 +10,7 @@
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/message_loop/message_loop.h"
 #include "printing/backend/printing_info_win.h"
 #include "printing/printing_test.h"
 #include "printing/printing_context.h"
@@ -159,6 +160,7 @@ TEST_F(PrintingContextTest, Base) {
 }
 
 TEST_F(PrintingContextTest, PrintAll) {
+  base::MessageLoop message_loop;
   if (IsTestCaseDisabled())
     return;