Force-generate a frame before the end of a layout test.

components/test_runner/test_runner.cc

Index: components/test_runner/test_runner.cc
diff --git a/components/test_runner/test_runner.cc b/components/test_runner/test_runner.cc
index 80162a192c626628a2b091ef94fb155bf999039f..3d54c9a4d7207ac349471ad6b6fcf69c38bcf991 100644
--- a/components/test_runner/test_runner.cc
+++ b/components/test_runner/test_runner.cc
@@ -2352,6 +2352,14 @@ bool TestRunner::GetAnimationScheduled() const {
   return is_animation_scheduled;
 }
 
+void TestRunner::RunPendingAnimations()
+{
+  for (auto& widget: widgets_with_scheduled_animations_) {
+    widget->updateAllLifecyclePhases();
+  }
+  widgets_with_scheduled_animations_.clear();
+}
+
 void TestRunner::OnAnimationScheduled(blink::WebWidget* widget) {
   widgets_with_scheduled_animations_.insert(widget);
 }