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 7ae37930300b8fe7d30ae12738fc68bdc5a06475..2a825854fc5139885897f9d24fe61b544e776770 100644
--- a/content/public/test/layouttest_support.h
+++ b/content/public/test/layouttest_support.h
@@ -143,6 +143,9 @@ 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_