Print preview: Use an ID instead of memory pointer string in WebUI.

BUG=144051

Review URL: https://chromiumcodereview.appspot.com/10870003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@153342 0039d316-1c4b-4281-b951-d872f2087c98
 void PrintPreviewUI::OnPrintPreviewTabClosed() {
  TabContents* preview_tab =
      TabContents::FromWebContents(web_ui()->GetWebContents());
  printing::BackgroundPrintingManager* background_printing_manager =
      g_browser_process->background_printing_manager();
  if (background_printing_manager->HasPrintPreviewTab(preview_tab))
    return;
  OnClosePrintPreviewTab();
}

 void PrintPreviewUI::OnPrintPreviewTabClosed() {
  TabContents* preview_tab =
      TabContents::FromWebContents(web_ui()->GetWebContents());
  printing::BackgroundPrintingManager* background_printing_manager =
      g_browser_process->background_printing_manager();
  if (background_printing_manager->HasPrintPreviewTab(preview_tab))
    return;
  OnClosePrintPreviewTab();
}
