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
SVN changes up to revision 251373