Always run the callback in CloudPrintProxyService::GetPrinters().

BUG=845250

Change-Id: Ifad55ab66e7c3bf99896a9be3dfc3c62a4e58832
Reviewed-on: https://chromium-review.googlesource.com/1178984Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584243}

chrome/browser/extensions/api/cloud_print_private/cloud_print_private_api.cc

@@ -109,8 +109,6 @@ ExtensionFunction::ResponseAction CloudPrintPrivateGetPrintersFunction::Run() {
   if (!service)
     return RespondNow(Error(kErrorIncognito));
 
-  // TODO(https://crbug.com/845250): CloudPrintProxyService::GetPrinters() may
-  // not invoke the callback, which means this function may never respond.
   service->GetPrinters(
       base::Bind(&CloudPrintPrivateGetPrintersFunction::SendResults, this));
   return RespondLater();

chrome/browser/printing/cloud_print/cloud_print_proxy_service.cc

@@ -136,8 +136,11 @@ bool CloudPrintProxyService::ApplyCloudPrintConnectorPolicy() {
 
 void CloudPrintProxyService::GetPrinters(const PrintersCallback& callback) {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
-  if (!profile_->GetPrefs()->GetBoolean(prefs::kCloudPrintProxyEnabled))
+  if (!profile_->GetPrefs()->GetBoolean(prefs::kCloudPrintProxyEnabled)) {
+    base::ThreadTaskRunnerHandle::Get()->PostTask(
+        FROM_HERE, base::BindOnce(callback, std::vector<std::string>()));
     return;
+  }
 
   base::FilePath list_path(
       base::CommandLine::ForCurrentProcess()->GetSwitchValuePath(