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

components/test_runner/web_test_delegate.h

Index: components/test_runner/web_test_delegate.h
diff --git a/components/test_runner/web_test_delegate.h b/components/test_runner/web_test_delegate.h
index 41da496b783b5864b45cc10c39ac953a08e46745..14e68ae4b376132e447dca920827de6228968601 100644
--- a/components/test_runner/web_test_delegate.h
+++ b/components/test_runner/web_test_delegate.h
@@ -278,6 +278,9 @@ class WebTestDelegate {
     const blink::WebPluginParams& params) = 0;
 
   virtual float GetDeviceScaleFactorForTest() const = 0;
+
+  // Run all pending idle tasks, and then run callback.
+  virtual void RunIdleTasks(const base::Closure& callback) = 0;
 };
 
 }  // namespace test_runner