web_apps: rewrite !callback.is_null() to use operator bool

Eliminate the unnecessary negation in if condition, because we can use
operator bool to check the callback is provided.

Change-Id: Ib21c24a4ecf37ca98191fbbe0f8bdf8d7dd52efe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2395001
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Auto-Submit: Jiewei Qian  <qjw@chromium.org>
Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804669}

chrome/browser/web_applications/pending_app_manager_impl.cc

@@ -218,7 +218,7 @@ void PendingAppManagerImpl::StartInstallationTask(
   }
   current_install_ = std::move(task);
 
-  if (!current_install_->task->install_options().app_info_factory.is_null()) {
+  if (current_install_->task->install_options().app_info_factory) {
     current_install_->task->InstallFromInfo(base::BindOnce(
         &PendingAppManagerImpl::OnInstalled, weak_ptr_factory_.GetWeakPtr()));
     return;