Add testRunner.runIdleTasks() to force idle tasks to run.

content/shell/renderer/layout_test/blink_test_runner.cc

Index: content/shell/renderer/layout_test/blink_test_runner.cc
diff --git a/content/shell/renderer/layout_test/blink_test_runner.cc b/content/shell/renderer/layout_test/blink_test_runner.cc
index 5d2f98a9482e51b64b047808fb36ffbecdf32655..99a88b2fa5d06e1453dcb4895dbf74a1ed470e92 100644
--- a/content/shell/renderer/layout_test/blink_test_runner.cc
+++ b/content/shell/renderer/layout_test/blink_test_runner.cc
@@ -701,6 +701,10 @@ float BlinkTestRunner::GetDeviceScaleFactorForTest() const {
   return render_view()->GetDeviceScaleFactorForTest();
 }
 
+void BlinkTestRunner::RunIdleTasks(const base::Closure& callback) {
+    SchedulerRunIdleTasks(callback);
+}
+
 bool BlinkTestRunner::AddMediaStreamVideoSourceAndTrack(
     blink::WebMediaStream* stream) {
   DCHECK(stream);