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

content/public/test/layouttest_support.h

Index: content/public/test/layouttest_support.h
diff --git a/content/public/test/layouttest_support.h b/content/public/test/layouttest_support.h
index 402fd044e164636ad7dd7172cde84c949ca3e039..be94cd49208dd75dd38a403650a866e61c3b8ccb 100644
--- a/content/public/test/layouttest_support.h
+++ b/content/public/test/layouttest_support.h
@@ -132,6 +132,9 @@ void DisableAutoResizeMode(RenderView* render_view,
 std::string DumpBackForwardList(std::vector<PageState>& page_state,
                                 size_t current_index);
 
+// Run all pending idle tasks immediately, and then invoke callback.
+void SchedulerRunIdleTasks(const base::Closure& callback);
+
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_TEST_LAYOUTTEST_SUPPORT_H_