mash: Speculative fix for content_browsertests hang

Speculative fix to single process mash content_browsertests
start-up hang by allowing nested task while waiting for display.

Bug: 892173
Change-Id: Ia146293d8f2553cd8233feae80786e56402c7c55
Reviewed-on: https://chromium-review.googlesource.com/c/1262925Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597150}

ui/wm/test/wm_test_helper.cc

@@ -100,7 +100,10 @@ void WMTestHelper::InitMusHost(service_manager::Connector* connector,
   if (running_in_ws_process) {
     // Spin message loop to wait for displays when WindowService runs in the
     // same process to avoid deadlock.
-    display_wait_loop_.Run();
+    display_wait_loop_ = std::make_unique<base::RunLoop>(
+        base::RunLoop::Type::kNestableTasksAllowed);
+    display_wait_loop_->Run();
+    display_wait_loop_.reset();
 
     // Bind to test_ws so that it could be shutdown at the right time.
     connector->BindInterface(test_ws::mojom::kServiceName, &test_ws_);
@@ -142,7 +145,8 @@ void WMTestHelper::OnDisplaysChanged(
     int64_t primary_display_id,
     int64_t internal_display_id,
     int64_t display_id_for_new_windows) {
-  display_wait_loop_.Quit();
+  if (display_wait_loop_)
+    display_wait_loop_->Quit();
 }
 
 }  // namespace wm

ui/wm/test/wm_test_helper.h

@@ -98,7 +98,7 @@ class WMTestHelper : public aura::client::WindowParentingClient,
   std::unique_ptr<aura::client::FocusClient> focus_client_;
 
   // Loop to wait for |host_| gets embedded under mus.
-  base::RunLoop display_wait_loop_;
+  std::unique_ptr<base::RunLoop> display_wait_loop_;
 
   test_ws::mojom::TestWsPtr test_ws_;