PlzNavigate: fix timing issue in app window creation

extensions/browser/app_window/app_window.cc

Index: extensions/browser/app_window/app_window.cc
diff --git a/extensions/browser/app_window/app_window.cc b/extensions/browser/app_window/app_window.cc
index b6d7754c56a47124255bc3365a36b470696500b1..7d84438aa58f9a6ed5b2cf9191705951739ca4bd 100644
--- a/extensions/browser/app_window/app_window.cc
+++ b/extensions/browser/app_window/app_window.cc
@@ -11,6 +11,8 @@
 #include "base/command_line.h"
 #include "base/strings/string_util.h"
 #include "base/strings/utf_string_conversions.h"
+#include "base/task_runner.h"
+#include "base/thread_task_runner_handle.h"
 #include "base/values.h"
 #include "components/web_modal/web_contents_modal_dialog_manager.h"
 #include "content/public/browser/browser_context.h"
@@ -442,6 +444,26 @@ void AppWindow::DidFirstVisuallyNonEmptyPaint() {
   }
 }
 
+void AppWindow::SetOnFirstCommitCallback(base::Callback<void(void)> callback) {
+  on_first_commit_callback_ = callback;
+}
+
+void AppWindow::OnFirstCommit() {
+  CHECK(base::CommandLine::ForCurrentProcess()->HasSwitch(
+      ::switches::kEnableBrowserSideNavigation));
+  WindowEventsReady();
+  if (on_first_commit_callback_.is_null())
+    return;
+  // It is important that the callback executes after the calls to
+  // WebContentsObserver::AboutToCommitNavigation have been processed. The
+  // CommitNavigation IPC that will properly set up the renderer will only be
+  // sent after these, and it must be sent before the callback gets to run,
+  // hence the use of PostTask.
+  base::ThreadTaskRunnerHandle::Get()->PostTask(FROM_HERE,
+                                                on_first_commit_callback_);
+  on_first_commit_callback_.Reset();
+}
+
 void AppWindow::OnNativeClose() {
   AppWindowRegistry::Get(browser_context_)->RemoveAppWindow(this);
   if (app_window_contents_) {