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 653855eff72b9ae1c92a763f2599ac4f961b65d5..1ce0bf9e744dc7f5f2255679b140ffd3e5d3fb39 100644
--- a/content/shell/renderer/layout_test/blink_test_runner.cc
+++ b/content/shell/renderer/layout_test/blink_test_runner.cc
@@ -704,6 +704,10 @@ 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);