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 ef5dddf4987d7c153e93d62254bd1b493a8d3604..59c210e3141064f857caaad82df85896490009bf 100644
--- a/components/test_runner/web_test_delegate.h
+++ b/components/test_runner/web_test_delegate.h
@@ -277,6 +277,9 @@ class WebTestDelegate {
   // Convert the position in DIP to native coordinates.
   virtual blink::WebPoint ConvertDIPToNative(
       const blink::WebPoint& point_in_dip) const = 0;
+
+  // Run all pending idle tasks, and then run callback.
+  virtual void RunIdleTasks(const base::Closure& callback) = 0;
 };
 
 }  // namespace test_runner