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 76f1fccc9ad57539cf0bcc5e9d0a4d413d2bc92a..371bad9cf73f10c5fbbae9b24dc84ea9f08cc35b 100644
--- a/content/shell/renderer/layout_test/blink_test_runner.cc
+++ b/content/shell/renderer/layout_test/blink_test_runner.cc
@@ -738,6 +738,10 @@ blink::WebPoint BlinkTestRunner::ConvertDIPToNative(
                          point_in_dip.y * scale);
 }
 
+void BlinkTestRunner::RunIdleTasks(const base::Closure& callback) {
+    SchedulerRunIdleTasks(callback);
+}
+
 bool BlinkTestRunner::AddMediaStreamVideoSourceAndTrack(
     blink::WebMediaStream* stream) {
   DCHECK(stream);